I would like to have a campaign where key battles between Good and Evil are fought over which key heroes change from/to one alignment set to another (one 'team' or cult is NE/CE/LE vs. another group is NG/LG/CG). In D&D, alignment changes can happen swiftly, especially with magic. Are there ANY hard-set rules on which alterations are stronger and dominate a mind more effectively over time? Are there any rules stating that any of these forces dominate above and beyond 'freewill' or individual choice?
Example of alignment shifting in game play:
A frog (CR 0 / 'neutral'... sort of) gains a True Polymorph and becomes an Elf commoner (CR 0 / some alignment and it suddenly MUST choose now - presumably CG?). This commoner is then soaked in a 'Red Dragon blood ritual' (DMG suggestion) and transforms (well, half transforms... technically) accordingly (now CE?). While in this state it is bitten by a wererat and, over the next full moon, gains a new alignment we think (LE maybe?). Confused, this creature visits Bitopia for a few weeks and becomes some version of 'lawfully goody neutral-ish' or something else entirely. Later on, whilst down a street on the Prime Material this guy is hit by Dispel Magic and is now back to being a simple frog (stupid & neutral... but 'still in character' apparently?). Fortunately someone nearby has both a Headband of Intellect and Speak with Animals handy and do Sentient First Aid® on this poor frog-guy-dragon-rat-thing from heaven. They can now ask this suddenly-genius creature what alignments it liked (assuming it can remember) and what it thinks is now. With 19 intelligence it answers with great clarity. Imagine how interviewing reporters are rapt with anticipation.
Please consider these points of interest:
Mr. Crawford pointed out in some tweet (cannot find this) that a demon or devil that is not of its proper alignment is no longer a creature of that type. This means that he gets that alignments are essential. That said, Fallen Angel is clearly something that exists in 5e (and is discussed somewhere in the various books / cannot remember where on this either).
Some external alignment change devices clearly have more pull than others. Going to various planes can shift an alignment but it reverts back with a spell. The pull of lycanthropy seems to be more of an instinctive-reactive approach and not a full requirement for being any specific were-creature.
Many spells that modify behaviour can be long-lasting or even permanent yet have utterly no impact on alignment. Note that Mass Suggestion and Geas-Quest can be really pervasive for any (N)PC and cause them to act in ways radically different from their origin-alignment. There does not seem to be any writeup in any canon or follow-up books (Yolo / Xanathar) that suggests this is of any concern other than a paladin would need to get the requisite Atone checkup after a few thousand of miles.
It is not clear what alignments are even relative to or what or why. For example, Mind Flayers / Illithids are listed as evil. Yet to one another they are kind, peaceful and very decent. They eat the brains of much more stupid creatures but humans do this with pigs and it is a non-issue. If a Mind Flayer were involved with some kind a humanitarian-PETA project would they be considered good now? Or possibly Evil for reducing the logical supply of food for her own kind?
There are many spells, magical items and situations that concern alignment. A relatively well-written guide can be found here.
Things to consider as a DM:
5E has relatively weak demands and distinctions as to what qualifies as an alignment or what is a 'change' thereof. Discussion of this in The Angry DM can be found here.
As a DM i fathom that this could entail Player Character Agency Problems (PCAP): it is game-suicide to tell players how to think, feel, act &/or otherwise control their choices. These good-evil transformation mechanics would work most strongly on NPCs. On players ('PCs') these alignment modifiers would be gentle 'role play suggestions' at best.