No, just no. There is little to no evidence in the game to support this theory, it is more or less an attempt to give a back story to a backgroundless villain.
You can quickly and effectively kill any thought of an R=U Theory with a couple of facts.
A. Sorceresses have normal life spans and the game clearly demonstrates a large difference of time between Ultimecia's time and the time of Rinoa's (An example of the large time difference would be how Ultimecia's speech is differentiated from our heroes "Kurse all SeeDs". It is much how english used to be spoken like this "What does ye want with thee?"). Rinoa would've died before the time of Ultimecia, therefore she isn't Ultimecia.
B. Ultimecia tried to kill Rinoa a couple of times, if Ultimecia was Rinoa, she wouldn't have done that because killing oneself in the past would also kill one self’s present.
C. There are no facts in game hinting toward any arguments saying that Rinoa could've lived for along time, or that Ultimecia somehow forgot about her past and would be pure speculation with no in game truth behind it.
