Download the DMG then you can use it on all machines etc.
As you correctly said, when you activate PitStop is tied to a machine finger print.
This is how I would do it.
Leave the macs that are being decommissioned alone at the moment.
Install PitStop 2018 (released today 24th April) and Acrobat on the new macs.
Activate them as a free 30-day trial. So you have the both the old and the new with PitStop available.
run the new machines for a few weeks to make sure everything is ok and you have copied all your old Action Lists and settings etc from the old machines.
Then when you are happy all is ok, deactivate PitStop on the two old macs, when you do this there is an option to save the license files. So save them.
Then use those license files you've saved to activate the PitStop's on the two new macs.