old GW system clean up and migrate to new tree

tldr; What's the best way to clean up an ancient GW system before upgrading to GW 18 and moving to a new edir tree?

I'm dealing with a Groupwise system that was born I believe at Groupwise 6, and has been upgraded  to 7, 8, 2012, 2014R2.

Now the plan it to go to GW 18, along with a new eDir tree with OES 2018.  I want to start as clean as possible, so before upgrading to GW 18 is there any merit in creating a second PO and moving all users to it?  Will that leave junk/orphaned files behind in the old post office, which could then be deleted?

Then the GW 18 upgrade could occur before/after moving the PO and Domain to a new server, and be re-associated with the eDir users.

I read that in GW 8, a move to a new tree would require grafting, but I don't believe that exists anymore as GW is a standalone directory that just needs associating with eDir objects for LDAP authentication.

Is this a reasonable approach or is there a better way?