There's no logical reason to have Replace Units available in some areas of the game and not others. It's MORE confusing to users to have Replace option on some battle windows and not others - be consistent!
It makes no sense why continent map, neighborhood battles, challenges and recurring quests and QI don't have Replace button, but PVP arena, GE, GBG do. If its about a time race factor and not "quality of life" for players, then GBG should be the only feature that has Replace. That's the argument made on Discord, that its about racing - but there's no race in PVP arena or GE and those got it, so it feels more like a capricious decision by a developer.
If this is about "quality of life" improvement for players, then be consistent and put this replace button on ALL battle windows throughout the game. There's nothing gained by not having the Replace units everywhere, but plenty to lose as I already resent the windows that don't have it because I have to have 2 mind sets and I still have to manually replace units in half the game. FFS - if you're making an improvement then make it, don't do it half way.