I don't know if it will work - but:
If both are portable, then you have the old installation in just a single folder so it should be easy to make a copy of it as backup, install the new version into the folder of the old version and then see if it worked.
And if it didn't: delete the new installation, continue to use the old one - or start from scratch with the new installation.
Here is a thread with more information about how to keep the individual settings:
Where do you see the difference?
I think there have been for a very long time the folders data and export - which contain the user data.
and in the data folder there are the folders actions, sources - an perhaps new: library and panels.
But as it is not advisable to use the old source, you only need actions.
And perhaps some of the ini files.
What happens if you install the new version over the old version?
What kind of filesystem is that that it allows 2 folders with the name data?
AFAIK mp3tag always had mp3tag.exe as program file. So I don't know what mp3tagportable.exe should be.
Could it be that we are talking about 2 different programs?
That portable version of mp3tag was created by PortableApps.com in a time before mp3tag was natively portable. Their system includes the original program in a sub-folder and uses their portable-named program to call the original (official) program with parameters to make it portable. This might include cleaning up the offending files/registry/etc that the program leaves behind. It also works with their platform to automatically update the programs associated with the platform.
As I said at the start, mp3tag is now natively portable. You don't need the version from PortableApps.com to enforce portability. If you use the platform, go to PortableApps.com to get their newer version. It will update mp3tag without losing any settings.