There is failover to DR site (assuming source site is lost with local primary backups for undetermined time)
I now need to perform backups on DR site
Can I re-use existing backup copies and continue the chain using DR VMs? As replica is seeded from backup chain, it should contain the same data block baseline and backuping replica VMs should effectively be just a new incremental backup. Some VM ID remapping might be required though...
I couldn't find any reference in documentation if it were explicitly possible but it would greatly reduce backup target capacity requirements if it were possible. Otherwise we'd have to create a new chain and that would possibly consume a lot of disk space.
I believe that it would be better to create a new backup chain from scratch.
Replicated VM has another reference at vSphere level than source one so this is completely new VM from backup application perspective.
I don't recommend to perform any kind of manual VM ID remapping because it won't be possible to predict potential issues which can occur due to this change.
Yeah, I didn't expect to be supported but it seems like a cool functionality to have and in my naive opinion it wouldn't be too hard to implement (tracking source and replica VM IDs, repointing existing job to replica VMs and backup copy chain; or create new job that could be mapped to backup copy chain.
Just FYI: A similar tool exists but it's useless in your case because it's not designed for mapping to backup copy chain.
Moreover, it takes name of source VM as an input argument but replica name is usually different due to suffix added. I don't recommend to use this tool.