Normally, backups are not downward compatible, however ... (I've not tried this myself and it's a theory if the stars/planets are aligned just right, it *might* work!)
In moodle_backup.xml header of an unzipped .mbz backup there are these lines:
<moodle_version>2013111806</moodle_version>
<moodle_release>2.6.6 (Build: 20141110)</moodle_release>
<backup_version>2013111800</backup_version>
<backup_release>2.6</backup_release>
IF one were to look at a backup from the 2.5 site one might be able to 'doctor' a 2.6 backup to be compatible with a 2.5 site restore. This would be dependent on how close the versions of Moodle are (changes to tables, columns, etc. in DB) as well as modules contain in the backup - i.e., standard/stock moodle mods/blocks, etc. vs any custom mods/blocks or add ons (some more 'doctoring' might be required before re-zipping, renaming to .mbz and attempting).
Wouldn't make this a practice ... only use on 'no other way' situation. ;)
Best of luck!
'spirit of sharing', Ken