Hi
from my point of view you can do it but there is one thing to think about and to be careful:
A VM FGT is using a license file. If you load the license file to FGT afterwards FGT comunicates to FortiGuard Cloud Service to register the VM with the correspoding license file and THE IP of the instance (using IP which is used by comunicating to FortiGuard). If you come up with the new Instance in the new datacenter and "YOU DID NOT SHUTDOWN THE OLD ONE" the new instance is asap deactivated and you have to open Customer Ticket to explain etc. From this point of view YOU HAVE TO BE CAREFUL that the old instance is NOT anymore active if you bring up the new instance even it is the same as the old instance (new location). If you look on this it should work out of the box and you can do it 3 times (innofficial information) until the license will be deactivated which means: There is something like a ticker on the license which allows you to change the IP which comunicates to FortiGuard 3 times afterwards if needed you have to open Customer Ticket to ask to reset the ticker. I have many customers which did such a move like you would like to do and as long you have the old instance not active at time the new instance comes up it should work out of the box.
hope this helps.
have fun
Andrea