If you didn't make any changes in your fork of the project, you can just update your fork to include the latest upstream changes and get the license change along with it.
If the copyrights on the changes made on your fork are all owned by you, and you agree with re-licensing those changes under the MIT license, then you can merge the upstream changes into your fork, including the license change.
If, as part of your changes, you added the use of a GPL-licensed library, then you can change the license of the code in your fork, but the combination with that external library would still be governed by the GPL license.
If you don't own all the copyrights on the changes in your fork, for example you accepted contributions from someone else or you re-used code written by another, then you must get approval from those others before you can incorporate the license change in your fork. As long as you don't have that approval, you must take care that you don't merge the upstream commits of the license change into (branches of) your fork that also contain GPL-licensed content for which you don't have permission to change the license.