I have a rotten subfloor on the second floor. There is radiant pex underneath. Can I replace the subfloor without ripping up the ceiling below?

If it is attached to the subfloor, I don't see how you can avoid it, at least to be able to re-attach the tube to the new subfloor.

Surprised that there was no evidence of moisture on the 1st floor ceiling to begin with. How did you determine you had a subfloor problem? Spongy area around a toilet or something? You might be able to avoid a ceiling tear out if it is a localized problem area around some type of plumbing penetration.

The radiant tubing may have been run to avoid, say, a waste piping penetration. I'd start with a small subfloor cutout to try and get a peek of what I was dealing with. Might get lucky, otherwise, at least some ceiling will need to come out.