Since the pain that was dealing with Picnic Flora, all TT+ bosses have gotten degrades to their kits, so yeah.
Imagine the nightmare dealing with NY!Plumeria with just Dance allowed.
It's a case by case basis, most actually don't get changed at all.
Just going from last year's halloween to now:
halloween grima: unchanged
ninja lyn: unchanged
winter sephiran: unchanged (they didn't even give him a b skill??)
new years plumeria: got rid of only dance
plegia tharja: base kit + a b skill, with Chill Res (sephiran left in tears)
Kralphonse: Alfonse's base kit, but with Guard. DB3 & Spur Attack were not upgraded to tier 4 or anything
spring myrrh: lost distant counter, kept everything else
kid erika: lost distant foil, kept everything else
wedding catria: unchanged
summer hilda: she got a balm added to her kit
pirate surtr: lost his menace prf skill.
So generally they look at if it becomes a problem and go from there.
Nifl & Muspell I left out because they're a little special, but let's take a quick look at them because Fjorm is most comparable to the two of them (a new unit previewed in a tempest for release later with "proper" skills)
Tempest Nifl: No assist. Draconic Aura. Swift Sparrow 2. Lull Attack/Speed. Joint Drive Res
Release Nifl: Rally Def/Res+. No Special. AR-D Attack/Speed 4 (new). Lull Attack/Speed. PRF skill.
Obviously the AR-D skill wouldn't be here, it'd be worthless, so Swift Sparrow is basically a placeholder to sort of emulate it (though a Push skill likely would've worked better). Her PRF skill, like all OCs, wanted to be kept until playable so she got one that referenced it.
No real logic to going from no assist/DA to double rally/no special.
Tempest Muspell: No assist. Ignis. Bracing Blow 2. Dragon Wall. Joint Drive Def.
Release Muspell: Rally Up +. No Special. AR-D Def/Res. Dragon Wall. PRF Skill
Similar logic, though the joint drive doesn't work as thematically.
I presume one of Dating Stance or the Chill will get replaced with a new skill (either a prf or a new tier 4 skill that's not in the game) but it's still not....good.....