Migrating from ostree to composefs backend

I don't think we have any reasonable, documented way to do this today. Just creating this as a tracker and a place to write down potential approaches and any known problems. We're going to need to be able to do this eventually, and selfishly right now I'm in a place where my laptop has migrated from ostree-based Silverblue => ostree-based custom bootc deployment and I would like to swap to the composefs backend but am also too lazy at the moment to back everything up and wipe to do a clean install.