Depends ... yes/no/maybe ...
What's in the 2.6.11 backups - mods/blocks that no longer exist in the higher versions?
Do the 2.6 backups contain users? Admin/User conflicts.
Quizzes - did the 2.6 have question types that no longer exist in higher versions ... even if
question types exist, are they still compatible?
One could extract a 2.6.11 .mbz file to it's parts in a test directory then look at the .xml files and related folders for some clues, but that's very time consuming.
Or one could simply try to restore and step through each screen of the restore. If you see a activity/mod/block/etc. that is no longer in the moodle you are restoring it to, exclude (un-check).
Afraid no one can really answer that question with 100% certainty ... you just have to try and find out! :|
'SoS', Ken
Thanks for the answer Ken always appreciated.