ULIDs technically fall under UUID v8 since they map onto the 128bit space. So it's possible that you can just keep using them and PostgreSQL might have something for v8 specifically.
I haven’t had my coffee yet but I’m pretty sure v8 still needs to set the version bits to specify its a v8 UUID, which means there’s 6 bits of data that need to be thrown away to migrate from ULID to a fully conforming UUID.