Different name. Different symbol. Different look. Different getup. Different attitude. They're only the "same person" on a technicality. It's like saying "Oh, we replaced Wakka with Barret, they're the same person though." We can only have an attachment to characters based on what we see, and so far, the two of them have appeared to be markedly different.

Again, in time, I'm sure this will be forgotten by many and most, or just be seen as a "remember when" kind of thing. But at the moment we need to see a lot more Luna before people will forget about Stella.

EDIT: Although we can't be sure Luna will do those same things considering they have taken out key parts of the scene. It's possible that general badassery won't be seen with Luna.

EDITRA: To drive the point home, quoting from a reddit summary of this ATR...

Stella was cut because the team had difficulty integrating her role into the game. Instead we have Luna now, who has a different role than Stella used to have. All the other characters presented so far (e.g. the black-haired woman) are still in the game.