I have done similar but from 5.6 to 6.0. Unfortunately FMG is screwed up on this. The only way to do this and not do anything completely anew is the way you wrote. Even TAC agrees with this but does not recommend it
When I did this I completely lost all interface mappings in FMG first. TAC found a way to re-apply a backup to get them back. Still this sucks majorly. Seems to be a case Fortinet did not really consider :\
The recommended way would be to remove the FGT from the Adom, upgrade it and put it into a new admon for the new firmare version. But this would require me to redo 100s of interface and address mappings and also it would create a load of useless policy packages. Also I would have to redo the complete default policy package for the new adom since you cannot export or import it. This is not what I understand in central management....
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