Re: Migrate from Fortigate 50E to 60E via Fortimanager.
Unfortunately this is not the easy way. If it is same model you replace with you can simple replace the serial in FMG Cli.
In this case you would have to apply the device config to the 60E. If FGT50E ad 60E have identical port names and numbers you could try to make a backup (Or download the last revision from FMG) from the 50E and also the 60E. Then replace the first 3 or 4 Lines (the ones with beginning with a "#") in the FGT50E backup with those from the 60E Backup. Then try to restore it on the 60E. With same Port names and numbers this cheat usually works.
Then add the 60E to FMG.
If you use provisioning template and/or a default policy package in that adom you should roll it out the the FGT60 afterwards.