I was up on the roof last year and noticed the mushroom caps on my vents were getting chalky and brittle, although not yet leaking. And the sealant around the bases was starting to crack. I bought new vents. The bases still looked good. Not wanting to remove all the sealant to remove the old bases, I just removed the mushroom caps from the new vents and installed them on the old bases. And I resealed everything on the roof with new Dicor while I was up there.
https://www.amazon.com/RecPro-Attic-.../dp/B017MP1RH0