What about doing reverse merges, back to the point in time you created the branch and merging the whole branch afterwards. This should end up in a mainline looking exactly like the branch line.
Take a look at: https://forum.mendix.com/link/questions/8380 ... please take note that this is advanced stuff don't do it alone.
Checkout the answers for this question: https://forum.mendix.com/link/questions/8380
I don't like the solution suggested in https://forum.mendix.com/link/questions/8380 as this will effectively delete part of your SVN history. I would go for renaming the directories on the SVN server directly.
There is no magic as to what's in trunk and what's in branch/foo. These are all just normal directories as far as you SVN server is concerned.
The only problem you might have is that the Modeler will try to determine the model version by looking at the metadata for the latest commit. So you may manually need to update this metadata as well.