Like, if in a future Harrowing event Gankplank (or any other champion) gets killed off, he would be re-released as a Shadow Isles champion? Or Quinn or Talon doubting the motives of Demacia/Noxus and run over to their respective arch enemies? (I know it would be most unlikely for the champions to do that; it's meant as an example only) Or Xerath trying to absorb the void, but getting assimilated by it? The list goes on.
Maybe on reworks, if they decide to update lores, like they did with Taric. But that easy, doubt it.
Technically it would be possible,but it's likely only gonna happen with reworks,since most champions design wouldn't really fit into other factions(like your example if xerath became a void related champ,he'd likely turn purple),unless it is something "minor" like volibear switching to ashes side.
What would be cool is Renegade Quinn + Renegade Talon - no affiliation to any city state?
Lorewise, look at Taliyah that went from Noxus to Indipendent/Shuriman.
Gamewise, ir already happens with skins. If you mean by a canon base skin, then it can happen if it makes sense and does not distrupt the character i guess.
Casting aside Trundle which is an edge case here i could see them change faction, but not too much graphically wise. You won' t see Yasuo donned with blue and gold armor yelling "Demacia" because that' s not the character you grown to, even if lorewise he sides with Demacia at some point. Probably he will recognize that with some additional VO or gain a decoration for that, but something subtle that does not distrupt the "air-lightweight samurai" style.
In fact that is really hard and is usually done with GU. Look at Taric for example: acclaimed and changed faction, changed drastically too but with a reason.
