Why did they change from Aeris to Aerith

[Sephiroth]

Aerith, the original name, is an allusion to "Earth" as the official Kaitai Shinsho says but Aeris is an existing word with a fitting sense so it was normal to think about it. Many names are just mistranslations like Bizarro Sephiroth who even is correctly called Sephiroth Rebirth on his artwork or Safer Sephiroth instead of Sepher Sephiroth, Fourth Bracelet instead of Force Bracelet, Reina, Salsa, and all. It is not the same as with Frioniel/Firion, Cain/Kain, Golbeza/Golbez, Butz/Bartz, Exdeath/X-Death, Tina/Terra, Mash/Sabin, Cefca/Kefka, Ultimecia/Artemisia, Eclair/Claire or others.

They still use Aeris for the newly released Final Fantasy VIIs like the Steam version as I've heard, however as many people are used to it and the names are in other programs as the normal textboxes.

I don't really have a preference as they're both very similar, and I alternate between the two. I find it very interesting that they're doing it with Aeris/th and not with the Tina/Terra issue in FFVI.

Because one is an error and the other is not. One was never meant to be two names and the other was. It is not the same case.