Well, the theory has barely stood it's ground for a long time now, and although it has been made quite clear over time that the game doesn't really support it, the final nail in the coffin has finally arrived, in shape of the Ultimania. This is what it has to say about sorceresses:

"SORCERESSES

Said to have existed from time immemorial to the present day, the sorceresses are women who are said to have received their powers from the old god, Hyne. There is, however, no hard evidence to support this claim. Extraordinarily powerful, many sorceresses have harboured ambitions to rule over the world – as a result, many people have come to equate the Sorceress with fear. However, there are also many Sorceresses who have chosen to live a quiet life sheltered away from civilized society; as such, the actual number of Sorceresses and the amount of power shared between them remains unknown.

The potential to become a sorceress is determined by one's capacity to wield such power – their natural affinity for magic. This factor helps to determine Sorceress candidates for when a Sorceress passes on all of her power into the next Sorceress. The giving and receiving of
power can be made between any two individuals – it not necessary for them to be related by blood. A Sorceress' lifespan is the same as a normal human's, however they cannot die until they have passed on their power to the next Sorceress."

---

Make special note that it says in the end that a sorceress has a normal lifespan. Now, it does say that a sorceress cannot die while she has her powers, but judging by Ultimecia at the orphanage, being forced to stay alive when your body is dead basically cripples you, and basically forces you to 'cough up' your powers to someone. This means that if Ultimecia were Rinoa, not only should she be an old woman, she should also be crippled by her inability to let go of her powers. These traits are clearly not present in the Ultimecia we see, so we can conclude that there is no conceivable way for Rinoa to be Ultimecia.

I'm sorry if this dissapoints believers of the theory, but this is official Square information, and it says black on white that a sorceress has a normal lifespan. Since there is no way to avoid the fact that Ultimecia lives too far into the future for a Rinoa with normal lifespan to 'become' Ultimecia, the theory is impossible.