I loathe the R=U theory and disagree with is wholeheartedly, but I can still answer a couple of the questions you pose there
Quote Originally Posted by champagne supernova View Post
The visual differences coupled with the Ultimania guide saying that sorceresses have a normal lifespan is a pretty strong argument. Ultimecia does not look much older than Rinoa. Even if Rinoa fell into a time hole (or anything you want to bring her into Ultimecia's time), at a normal rate of aging, such changes would not occur. Especially when their eyes are completely different colours!
The Ultimania explains that prolonged use of dark magic can physically distort a person - it's the reason Adel was huge and off-colour. It's also why Edea had odd eyes, long fingers and veiny growths on her temples. Those disappeared after she lost her powers and Ultimecia's possession. So in theory, if Rinoa turned 'evil' she might end up looking a bit different.
And a complete personality switch can be used, but normally, the writer will build up this personality change without an event. Do you think any half-decent writer would write a story with a sunny, bubbly, optimistic character, who lives in the moment, and then, without any warning or build-up, turn her into the complete opposite: depressed, pessimistic, so afraid of losing somthing that she's to scared to live, a loner.
A popular 'theory' is that she turned bitter after losing Squall, and perhaps forgot her past because of GF usage. According to Edea, having a knight helps to keep a sorceress honest and good or something. But except for Ultimecia being associated with the GF Griever, there's nothing that even remotely supports these hypotheses about Ultimecia.