That license change would only apply to the changes and additions, any of the original code would still be MIT licensed, also if the changes are trivial then the whole re-license might fall through.

Copyright would still lie with the original authors as well, after all sub-licensing allows you to pass on the rights that you already have, but does not give you any new rights.

Since the MIT license is mostly a super-set of the GPL you will lose some of the MIT parts if you sublicense but if you wish to gain more rights than what the MIT license already provides you with it will likely not work when tested in court. The best way to deal with stuff like this is to stick to the original license unless you have consulted with an IP lawyer that signed off on your plan, it is very easy to get these things subtly wrong.