As far as I remember the MPL, once issued, still lets you add another type rating, so you’re not tied to Flybe or the Q400. The only reason for converting to a full CPL after the licence is issued is for single pilot privileges.
the problem is likely to still be finding a company which will employ you, as some are still a bit untrusting of the MPL. I flew with several pilots who had an MPL from Sterling after it went bust, and they had no issues changing company, albeit on the same type.