Either the CPSB-MNTR can be added to their existing license or they can trade in their existing SMS license for a new one that has CPSB-MNTR in it. It would be worthwhile to check the cost of doing it either way and see which one is cheapest, also keep in mind that brand new SMS licenses typically include the Compliance and SmartEvent blades for 1 year. Gaia vs. Windows for the SMS OS doesn't matter for licensing.
For Smart-1s vs open hardware for a SMS the licensing does look a little different, but even the smallest SMS license for open hardware (CPSM-NGSM5) always includes the CPSB-MNTR blade now.
One other thing I forgot to mention, if you have a self-managed Check Point appliance (whether standalone or Full HA) the CPSB-MNTR blade is still not included and never has been. It can be added of course, but only a new SMS license in a distributed setup will have the Monitoring blade included.
--
My Book "Max Power: Check Point Firewall Performance Optimization"
Second Edition Coming Soon
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com