No, you cannot "just" change the license to GPLv3.
The MPL2.0 is a per-file strong copyleft license, with an opt-out provision to incorporate MPL-licensed files into a larger project that is under the GPL license. This means that if you only made changes to files containing MPL-licensed code, then you cannot change the license.
Also, if the author ...
MongoDB's SSPL license is not an open source license and is incompatible with the GPL, so it's good that you ask. But most likely, this license has no consequences in your scenario.
The SSPL triggers license conditions when you do one of the following:
you give someone else a copy of the SSPL-covered software
you create a derived work of the SSPL-covered ...