You can’t. Migrate, that is. If you’ve moved all your on-premises data from one server to another, your only option is to install the MARS agent on the new server, manually configure the new server, and re-upload all the (same) data again. Then change the schedule on the old server to stop new backups but retain old ones (you need to do this separately for System State and file backups).
Big plus for CrashPlan here which has for years supported “adopting” a computer.