Migrate from OS X 10.9 back to 10.8.5
Solution 1:
The easiest way would be do a Time Machine backup of your 10.9 Home and Applications folders only.
Then do a clean install of 10.8.5 and boot up in recovery mode and restore your 10.9 Home folder, and pick which applications you wish to restore from your 10.9 Applications folder.
I've had to do this rolling back to previous OS X versions and have rarely had too many problems.
Solution 2:
If you have a full backup of Mountain Lion from before the upgrade
Perform a full restoration.
If you do not have a full backup of Mountain Lion from before the upgrade
There is no migration (no downgrade) from Mavericks to any lesser operating system. You could:
- backup
- install Mountain Lion
- from the backup, manually copy or restore selected non-system items.
Critically
Some data from Mavericks may be unusable with Mountain Lion.
If, for example, the database for Mail was upgraded by Mavericks then you should not expect that database to be reusable with Mountain Lion.