You likely have no option about upgrading Pro Tools, unless you want to purchase a used Mac.
There is basic planning you can be doing around what differetn mac optiosn give you for running different Pro Tools versions.
Start at
https://everymac.com/systems/by_capa...supported.html
e.g. 2019 iMac 5K minumum version supports macOS 10.14.5 Mojave as the earliest (meaning you cannot install an older version).
So to work out if you can get a supported system....
Then look at
http://avid.force.com/pkb/articles/e...tibility-Chart
So Pro Tools 2019.6 and 2019.5 are the only officially supported versions on that mac, and in practice I'd go for that. Given the huge improvements in plugin processing (reduction in AAE CPU errors and improvements in scaling) in 2019.5 and later you probably want to be on those anyhow. BTW I think that combo of 2019.x and Mojave is one of the *best* combos in many years.
You'll be facing a ~$299 re-up/license update plan for Pro Tools, or switching to annual subscription.
What you may *not* want to do is wait out a future hardware update on these iMacs that will likely be locked to macOS Catalina and a possibly painful wait for support for that in Pro Tools.
You also need to work out if you have or can can get compatible plugins (compatible with 2019.x and macOS Mojave), etc. need to update/update/pay for stuff there.