Hi! Ou this is interesting. I mean like you said, ultimately it's your story so you have every right to do with it what you want. But I would imagine there's probably a lot to the original version that wouldn't apply to your new fandom? And probably, also, your own writing has undergone some growth if enough time has passed? If it were me, I know it would be the kind of thing where I'd probably do substantial rewrites just to make it work to my current-day writing level and to the new fandom, so the fic wouldn't be close to the same. But yea idk what you were planning on doing! Maybe disclose as much, or delete the older work--probably you're coming at it from an angle of "oh this would really work for this pairing in this other fandom" but some people might not see it as such.

I'm sorry if this hasn't been that helpful; I've not been in a position where I thought some of my original writing or so would work in a fic context!