You might want to check out the more recent posts in this thread:
Consolidated "Switching SWU/CR-1/Miles-Supported Upgrades to e500s" Thread [Merged]

Even though it says e500s there's info on swapping between the others as well.
I haven't read through it in a while, but I think the short version is that it's doable but tricky, to avoid losing your place in the UG line or the confirmed UG itself (depending on if it has cleared or not).

I'm sure others will be along with further details.

Welcome to FT btw. ^