No, when you break open the system for a major upgrade or retrofit you will need to purge the entire system. If the near boiler piping was installed correctly you should have some drain valves and a gate valve installed in the piping on the return and the supply. This is where the purge for the whole systems needs to be done if those valves and drain valves were installed. The air purger is for operational purging and not for initial fill purging and this is what you will have to do when break the system open like you are planning.