Or maybe the koroks are comemarating her by naming the lake after her unaware of the fact she became Hestu.
For the personality change Saria could be almost in hiding after realizing that a Link too was in this new Hyrule.
Or maybe she just doesn't want any trouble after all her time being the sage of forest in OOT.
Sounds like we are getting into some Doctor Who territory here. Not the same person, yet is.
Though, even with the idea that Saria could be hiding, it doesn't match Hestu's personality. Saria, who ever she would be pretending to be, would be wise, with ages of knowledge. And, if the reason for her deception is Link, she would likely keep her distance, physically and emotionally. Hestu, on the other hand, is very interested in Link's aid, and seems very childish (young, naive, and not very knowledgeable).
Nothing against anyone who wants to believe they are the same, but there simply isn't enough here for me, and what is here, is contradictory to the idea. New sage? Sure. I could even go with a reincarnation connection, seeing as how young Hestu acts. But not the literal same person.