You need to upgrade the mainline to 9.12 in order to merge those changes. If you are trying other way around it will work, when you try merging from 7.23 to 9.12 it will work. Once upgraded from merge options select Advanced Merge and select the versions you would like to merge to mainline from Branch line.
Hope I answered your question,
It Can’t be done please refer the below Image as your mainline also needs to be upgraded to the 9.12.4 and then you can merge your branchline to mainline.
I currently have the exact same use case with my team.
Updating the main-line to a higher version first as suggested is not possible. Upgrading to a higher version in complicated apps can be a big undertaking. Multiple commits, extensive testing and breaking changes are encountered during an upgrade to a higher Mendix version. Therefore this whole process can only be executed safely in a branch-line. After everything is working correctly this (higher version) branch-line needs to be merged into a (lower-version) main-line
There is a trick with the SVN-folder that works fine for updating to Mendix 8. See post: https://forum.mendix.com/link/questions/8380
However, this trick does not work anymore if you want to upgrade to Mendix 9. I imagine a lot of teams having trouble with this use case so I'm interested to see if anyone can come up with a new tip or trick.
you can not merge higher version branch line to lower version main line or vice versa.
Suggest you to use same modeler version for merging.