Well, the release notes don't seem to tell anything specific about it, but normally, if one wants to keep everything untouched at an upgrade test, he can backup everything before updating.

I didn't see that message (I also didn't get to update yet), but it seems that yes, the only choices seem to be that you either keep using 4.2 (I see no real reason for going back, actually) and keep your labels, either go back to 4.1.5 by recovering from seed and lose those labels.