Have you tried updating your mx6 branch to mx8 and then merging? Or does that lead to a conflict?
Merging or converting stuff from 6.x to 7.x or 8.x fails when your Version of 6.x was released later than 7.x or 8.x. That Happens very often with Bugfix releases.
For example you can't merge from 6.10.17 to 7.23.7
What you can do is to merge to the latest 7.x Version first. From here you can merge to all Versions of 8 with a later release than your 7.x Version. Maybe you need to upgrade your 8.x branch.
If that's Not an option, it could help do downgrade your Version. But there is no guarantee that this will work and that it does not destroy something.