Hey,
Did change the Mgmt servers/IPs couple of times, and my advise is to do your homework and sketch your plan.
Like Val said, is recommended to get a new Management machine and export the data from the old one to the new one.
You can get a demo license until you transfer everything.
The change of the Mgmt IP is not an IP change, as the GW's are tight to that box in order to get policies, updates, etc.
So in order to get a new manager, I would recommend to take it step by step and move a GW or cluster at a time and see that all is OK, prior to stop/delete the other old one .
Thank you,
PS: if you come with a plan we can say what you missed and guide