Well, Savin responded to a topic about this by saying that
TFs are by and large cosmetic, which seems to suggest that a shorter-lived character TFing to a longer-lived species wouldn't extend their lifespan any. A pantheon TF might do it since one of the gods is taking your soul and reshaping you at a fundamental level. Even if that's the case it probably only half-counts since it's not extending your Champion
as you've created them as the transformation is kind of a part and parcel of it, unless your ideal for that character just happens to be one of the potential pantheon TFs that we know about (and only if others get implemented in-game). Other than that, I'm sure that if you want to mentally roleplay you can imagine that the portal magic did something to your Champion's lifespan if you want them to live longer but I don't think there's anything established in-universe that would do it, at least not right now.
And yeah, I'm with you on the Ryn thing. I mean, I'm sure she's aware that she's likely to outlive any non-elf friends she makes but it would be sweet if there was some way for the Champion to not die of old age probably a century before her.