Enabling standby node Wal streaming without pg_basebackup

So currently i'm planning to do a migration to RDS where i'm using a snapshot of our master and doing a pg_upgrade on a new machine(reasons being so that if the upgrade fails for whatever reason, we still have our old postgres instance around) For good practice, i'd like to have a hot_standby as well. The problem is that pg_basebackup takes too long. I tried to be clever and took a snapshot of our standby instance(the original setup has a master and standby setup), did a pgupgrade and attempted to make them communicate to each other, but failed. Is there a way to do what i'm trying to do, or is it impossible? thank you