It’s a safe bet that she is a different Yar if looking at it from a multi-verse perspective.
Timey-Wimey universe shenanigans aren’t exactly consistent in Star Trek. TNG was still using the concept of “altered universes” instead of “alternate universes”. Alternate Universes wouldn’t really become a story-telling device until DS9 mirror-universe and on from there to Enterprise, Discovery, etc…
But when TNG was being made, unless I’m forgetting something, the premise was very much that THE universe of the show was altered, not that they were moved to a different timeline.
So from the perspective of the episode, yes…that very much was their Tasha Yar since it’s presumed it’s the same universe, just altered. If the episode had been aired after they had established the multi-verse, than it’s likely that she wouldn’t be the same Yar.
Her change in demeanour is likely an acting choice by Crosby, since she’s playing a Yar that has been on the Enterprise for much longer. She’s more comfortable and confident in her role.
OMG I’m an idiot! I completely forgot the O.G. series…
I have to absolultely turn in my nerd card. Seriously…I’m embarassed. Ignore me.