Just thinking. Can you make a fork of MongoDb before license change and keep merging upstream changes under former license? Not asking because I want to hurt Mongo, just curious.
Yes, you can fork, but no, you can't merge new changes, those come with a different license (except maybe for third-party contributions, you could ask them to double license them under the old one too).
> keep merging upstream changes under former license?
Upstream changes made by the project sponsor would be expected to be copyright and released only under the new license.
Unless the copyright holder permitted it (perhaps via the new license), it'd be copyright infringement for anyone to distribute those upstream changes under different terms.