note that FreePBX is only 1 component of Elastix. They currently have a release up to 2.11 in their repos for their Elastix 4(? whatever their latest non MT release is) The FreePBX backup only typically handles FreePBX stuff though you can add other files and databases.
You can try a backup and restore between versions then try and reinstall every module but the odds of success are low and you may leave some ghosts in the machine.