When GPL v3 was released, there was a discussion about relicensing (or dual licensing) the Linux kernel from GPL v2 to GPL v3 because the licenses are not compatible. This is very similar to what is proposed for MAME, so it might be worth digging around in the Linux kernel mailing list archives etc., and read the discussion. I think they ended up facing the problem that in order to change or add licenses, they need permission from every contributor (or their estates for deceased contributors).