@Praxis: You will generally lose it, mainly because the new "QMC2-MAME" wouldn't know from where to migrate... however, system-favorites can easily be merged manually (they are just text files)... software-favorites were part of qmc2.ini in the past and are meanwhile auto-migrated to the "user-data DB", but only on demand, that is when a system's software-lists are loaded. So unless you manually migrate software-favorites from qmc2.ini and/or the user-data DB (sqlite3 is your friend here, see the mess_softlist_user_data table in mess-user-data.db) you will lose them.

I don't think I'll find the time to add some migration tool until the next release.

A mind is like a parachute. It doesn't work unless it's open. [Frank Zappa]