Hi Sandy,
Oracle keeps controlfile in 3 locations and all these files will have same timestamp when DB is running.
Incase we change/replace the controlfile while DB is online, this replaced control file will have different timestamp than others are result in error.
So to replace a controlfile you need to make DB and SAP down in-order to maintain same timestamp across 3 locations.
Hope this helps.
Regards,
Deepak Kori