The backup agents installed on the DCs being backed up must match the version of RMAD that you are using. If you try and backup a DC that has an agent that is a different version than the RMAD server you will get an error similar to the following:
"Failed to connect to backup agent: The backup agent version xxxx has been detected. The agent version yyyy is required"
The only way to do a phased approach would be to stand up a new server and install the new version of RMAD on it. Continue to use your old RMAD server for your daily backups and as you upgrade the agents, add them to the collections on the new server and use the new server to back them up. If doing this you need to ensure that as they are moved to the new server, they are removed from the old server to ensure there are not 2 backup attempts at the same time and coming from 2 different RMAD servers.
The disadvantage of having 2 servers is, depending on where you store the backups, you can end up with the backups split between the 2 servers.
