Good day, gentlemen!
I have a fork of the project, the code of which is released under GPLv3. I release modules written by me under MPL1.1. Do I have the right to change the license for other modules from GPL to MPL if I made a significant part of the changes in them (of course, referring to the first author)?
The copyright holder, in this case the author, has the right to change the licensing terms for new releases. However, since this is a derivative work, all authors, regardless of the contribution, are the copyright holder, so general agreement is necessary.
In any case, you cannot change the licensing rules for releases already released under the GPL, they still require source code to be provided, and, accordingly, anyone can make a fork.