We have previously discussed how and why moveset changes happen, but we haven't discussed the full extent of how things work. So, let's get on with it!
There are three major change movements. These are:
a. Cross-generational changes,
b. Remake learnset updates,
c. Third Game Egg Move updates.
We will discuss how these major trends occur.
Cross-generational are the most obvious changes that occur in Pokémon franchise. The reason for this similarly obvious, as each generation updates existing moves and creates new moves(and in rarer cases, types), they create a wider reach of opportunity for new Pokémon. However, these are not the end. The cross-generational changes can be defined as four major routes:
i. Introduction of new moves,
ii. Introduction of old Tutors/TMs,
iii. Introduction of updated moves,
i.Introduction of new moves:
This is the most common variant. Newly introduced moves are given primarily to new Pokémon, but certain older Pokémon can get some moves.
Gen II: Ancient Power, given to Corsola, but also the older Fossil Pokémon.
Gen III: Cosmic Power, given to Solrock/Lunatone/Baltoy, but also to Staryu and Clefairy.
Gen IV: Discharge, given to Shinx-line/Pachirisu, but also to Pikachu and Magneton-line.
Gen V: Flame Burst, given to Pansear-line/Heatmor, but also to Charmender-line.
Gen VI: Grassy Terrain, given to Flabébé-line, but also Bulbasaur/Exeggcute etc.
As a general rule, these additions are not fully comprehensive. These additions generally randomly pick specific Pokémon to buff, even if similar Pokémon might also benefit from the same move. As time goes, these moves are added to more varied Pokémon.
i.Introduction of old Tutors/TMs:
This is the most cynical variant of move additions. In each generational shift, Pokémon gain or regain their Tutor moves, generally as Egg Moves. Some Pokémon that didn't originally gain the moves might gain them through the generational shift, for example Lillipup-line didn't have access to After You in Gen V, and only gained access this generation as an Egg Move, but generally the Pokémon that were capable of learning the Tutor in the previous generation inherits it in the next generation;
Gen II: Razor Wind, originally available as a TM in Gen I, but made available to Totodile as an Egg Move(removed post-Gen II).
Gen III: Dragon Breath, originally available as TM in Gen II, but made available to Feebas as an Egg Move.
Gen IV: Fury Cutter, originally available as TM in Gen II and Move Tutor in Gen IV, but made available to Buizel as an Egg Move.
Gen V: Earth Power, originally available as Move Tutor in Gen IV, but made available to Drilbur etc. as an Egg Move.
Gen VI: After You, originally available as Move Tutor in Gen V, but made available to Swirlix as an Egg Move.
This is sometimes an effective means of keeping lost moves within the movepools(such as Ally Switch and Natural Gift) but it is far from perfect.
iii.Introduction of updated Moves:
This is far rare than the previous methods, as moveset updates are given to benefit existing Pokémon that can learn the said moves, rather than to upgrade the moves to fit various different Pokémon. If the latter occurs, or if the former is drastic enough, this might occur. For example;
Gen II: Fairly limited to specific cases, such as Fury Attack, which was given to Rapidash, Lick, which was given to Lickitung, Bite which is given to Omanyte/Omastar etc.
Gen III: Also fairly limited to specific cases, such as Spikes, which was updated to stack up. Spikes was given to Skarmory and Omanyte.
Gen IV: Due to Physical/Special revolution, there has been a few changes, but these have not occured cross-generationally. Flame Wheel for Ponyta-line in Platinum is a notable example. Other updated attacks such as Beat Up and Heat Wave were already available to their respective Pokémon.
Gen V: Also fairly limited, most notable one is Rock Blast for Shuckle.
Gen VI: Also fairly limited, most notable one is Aura Sphere for Squirtle.
These updated attacks don't have immediate effect on the generation, but they become more important for intergenerational changes. We'll discuss these later.
Of course, the bulk of the cross-generational changes simply don't have any set behavior. Many, if not the majority of changes occur for sake of having a change. However, there are some trends within these changes as well. These are;
a.Bias towards STABs.
b.Bias towards orientation(physical, special, mixed)
c.Bias towards diversity
These three principles are the guiding force of all movesets, except maybe joke ones like Unown.
However, "others" can also be affected by general trends within the generation, such as a move being made widely available to a specific group. Some major examples include;
Guard moves; which are made available to most Fighting-types.
U-turn: which is made available to most Flying-types.
Electric moves: which are made available to most Psychic-types.
However, there is also a nastier principle that rules Egg Moves. This nasty principle is known as;
Special. Chain Breeding Principle:
Since I can't detail the rich and "proud" history of Egg Moves, I can only explain the primary reason why chain breeding exists.
Guides, and frustration.
Chain breeding, like many other iterative mechanics in most Nintendo games, is a nasty, labyrinth-like structure that is designed specifically to be as hidden and obscure as possible. As such, most members of breeding chains are not given chain moves because they make sense, but rather because they have dual Egg Groups. Some dedicated breeders like Spinda are only glorified because of this, but others are not that dependent on chain breeding for their success.
Chain breeding chains often occur thanks to cross-generational changes, as they are Egg Move focused, but they can occur regardless of the nature of the Egg Move change.
b.Remake learnset updates:
One of the inter-generational change methods, remake learnset updates are updates in three dimensions;
a.Level-Up moves(which are the primary changes)
b.Egg Move changes(which are rare)
c.TM changes(which are fairly recent)
These changes generally occur for Pokémon that are;
a.Either part of the Regional Dex
b.Part of the post-game.
HG/SS provided changes for Johto Pokémon, legendaries and Hippowdon, which were available in HG/SS.
We will now discuss how these changes occur:
This is the most common variant of inter-generational change. This occurs primarily because the level-up moveset of the Pokémon was originally created for the previous game, and is now unfit for the current generation/game.
These changes generally prefer either newly introduced moves in the generation, for example;
Gen IV: Weedle, who gained Bug Bite in Platinum.
Gen V: Pinsir, who gained Storm Throw in B2/W2.
Gen VI: Volbeat, who gained Play Rough in OR/AS.
Other times it can be a move that is related to the Pokémon's design, such as;
Gen V: Beartic, who gained Aqua Jet in B2/W2.
Gen VI: Girafarig, who gained Nasty Plot in OR/AS.
Other times it can be tutor move in that game, such as;
Gen IV: Squirtle, who gained Iron Defense in Platinum.
Gen V: Eevee, who gained Covet in B2/W2.
Gen VI: Lileep, who gained Giga Drain in OR/AS.
b. Egg Move changes:
So far, remakes only provided Egg Move changes if they were the last game of the generation. Much like the previous change, these often focus on the regional dex and available Pokémon, but this will be discussed in detail in the next section.
Another variant of this has happened fairly recently with OR/AS, though, with Plusle & Minun gaining new Egg Moves to balance their movesets.
c. TM changes:
As for Platinum, TM changes generally occur if a TM move is added to level-up scheme, for example;
Gen IV: Kricketune, who gained Taunt in Platinum
Gen VI: Sableye, who gained Quash in OR/AS.
However, other kinds of independent TM changes have occured in B2/W2 and OR/AS;
Gen V: Klink, who gained Wild Charge in B2/W2.
Gen VI: Delphox, who gained Dazzling Gleam in OR/AS.
These occur very rarely, so they are not a central point in moveset changes.
c. Third Game Egg Move updates:
The last big update in any generation, "Third Games" often focus on Egg Moves above other moveset changes. This is because last games of the generation often bring a breeding change, whatever that might be. Examples for this include:
Gen II: Crystal, which provided a large amount of Egg Move changes, such as Crunch on Ekans(not retained).
Gen III: Volt Tackle, which is given in Emerald to Pichu.
Gen IV: HG/SS Egg Move changes, such as Power Whip and Sludge for Bulbasaur.
Gen V: N/A.
Since these changes are rather obscure and not well established, we can't say anything about them, outside of the fact that they follow the rules of other Egg Move changes, and as said above, primarily focus on the domestic stock of the Pokémon of the game, whatever they may be.