Define data loss!
First an upgrade of OBM from 10.63 to 2020.05 (be aware to read and execute the upgrade documentation carefully and dont wonder if it take several hours) will not break the BSMC.
The BSMC is just an OA Agent which sends events. so the Agent will buffer the events during OBM Downtime. if SCOM generates a lot of events its may be a good idea to configure the agent event buffer size to maximum. More complicated is topologie or metric data.
I have not done this with SCOM integration but with other integrations. in case OBM Server is not available the OA agent of BSMC will buffer events so there is no data loss.
Different is the upgrade of the BSMC and the SCOM Integration package. The Upgrade of both of these packages may cause events got lost. You need to read the documentation carefully. if its an uninstall the old and install the new and reconfigure than tehre is a high change of event loss.
Topologie is on first sight no issue as these get reread from SCOM, BUT there is a pit fall. discovery polices do mostly "delta detection". means that opbjects that no longer exist gets deleted from RTSM. This can cause if first discovery isnt successful that all previous discvovered objects of this integration gets deleted!
so a ver good idea is to disable discovery policy and remove the agtrep.xml before the upgrade. so no objects got deleted until new discovery works properly.
A customer of mine run into the problem that Vcenter was down for maintenance so after a few hours the integration for VCenter starts to delete all previous discovered CIs and caused that polices got removed from all affected servers. also the assignments got lost. so a lot of crap happend. luckily they do a vmware snapshot so they were able to go back. RTSM allows to disable CI deletions. so this is an option to consider during the upgrade BSMC and integration.
metrics i cant give ou no advice as i have no clue how the SCOM Integration packages handles them.
so your decision to upgrade OBM or BSMC first, or both at the same times will depend on the SUMA.
If the current BSMC and the SCOM integration packages support OMi 10.63 in my personally opinion i would upgrade BSMC first.
Then OBM on the next run.
Good luck!
