I think when you got the maintenance contract you probably did an 'upgrade' from your previous license. And when you upgrade from one version of Pitstop to another the old version license is no longer good. At least that has been working for us since version 10 to 11 to 12 to 13 to 2017 and I assume 2018. For example we have an 8 user license of Pitstop 2017. With our old product key we can still run Pitstop 13 or Pitstop 2017 but only a total of 8 licenses so any combination of 13 and 2017 that adds up to no more than 8. At least that is what I think it is. Not 100% sure though.
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.
It definitely won't be like the 2017 version with regards to getting you guys license keys. They should be on their way IMHO.
If you want to get on 2018 now then the best way would be just to use the in-built 30-day trial key until your official key arrives.
I'll investigate internally and find out why these are not sent out in advance so they are with you beforehand.