The contents of the edited-and-not-saved tabs (which includes all unnamed tabs) are stored in the backup folder of your portable. So you would need to grab the backup folder from the v6.9.1 portable location, not just the sessions.xml, and put it in the v8.7.4 portable location as well.

As a caution to you: with how old v6.9.1 is (March 2016!), there may be too many differences to expect to just copy some things and for it to work. The configurations have changed enough that there may be incompatibilities (and I don’t even remember whether Notepad++ still did the temporary unsaved-files storage the same way back then).

When you upgrade Notepad++, the safest plan is to always make sure all files are saved to known locations on your computer (actually, the safest plan is to always make sure all files are saved to known locations on your computer, whether or not you are about to upgrade, but I digress) – if you still have the folder for the old version (since it’s portable, I hope you do, especially since you had trouble with the changeover), then you could, in theory, go back to the old copy, save everything to a known location, then copy over the session.xml from your old copy to your new copy.