Hello,
First, only unregister UCS Central if you do not plan on registering it again with the same UCS domain.
Are you using Global policies or is everything being managed locally?
When you remove UCS Central from UCSM, you opt to "Localize Global" which will make all global policies local objects or you "Deep Remove Global" which will remove all global objects and policies.
If you do not have any service profiles that are relying on global objects, they should not reboot. If you have global objects, you can localize them and then clean up whatever is unnecessary after the removal.
Let us know if you have any other specific questions. If possible, you can open a TAC case and have the configuration reviewed for any potential issues.
HTH,
Wes