In the installation folder of your modeler, you can find a folder called “modeler”. Inside of this flder, there is a program called MprTool.exe
You can use it to view and change the meta information in a model package. It can be used to change the version. But you need to be carefull with that. A downgrade can work, but it can also fail. I have used it once and it worked. i downgraded between two Mendix 6 versions.
Create a backup and try on your own.
That should be possible by doing a reverse merge (https://docs.mendix.com/refguide7/using-version-control-in-the-dm#7-2-3-reverse-merging)
together with restoring a backup of your data of before the upgrade.