I finished a MDS R80.10 to R80.30 migration a couple of months ago and last weekend in a second attempt I migrated a 3 set of MDS servers from R77.30 to R80.30 exactly with that method:
Build a new server with R80.30, give it a temp IP
When you are ready to go this is what you do:
stop the CP services (mdsstop/cpstop)
start the migration export (use the upgrade guide)
once done change the IP of your original management server(s)
change the IP of the new machine to the original IP of the management server
start the FTW and finish it
move the export file from the old to the new server
use the commands from the manual to import the exportfile
start the cp services (cpstart/mdsstart).
run your check ups to make sure everything looks ok
install database (on every domain)
install policy(ies)
check logging.
Regards, Maarten