Linux Flavor locked at incompatible version and can't be upgraded to another version

Years ago we made the mistake of buying a Precision Desktop machine with Ubunto Linux. We have an old piece of software that runs an Nuclear Magnetic Resonance (NMR) system which required an older version of Linux. Unfortunately after many futile attempts, we were unable to install the softqqware because the OS was locked at the factory into the newer version of linux. Because we have had extemely good luck with Precison T3500 boxes with all of them, purchased ca 2012, continuing to run RHEL 6.0 , we are ntertaining buying another, maybe more of the newer Precision versions of Precisions.  The one I bought in ~ 2019 with ubuntu locked in was relegated to a data backup workstation and we continued to use old the T3500 box for that workstation to this day.

My questions are, can Precision workstations be acquired without the OS locked in from the factory? How do I order them without an OS or with an OS that is upgradable to another Linux version that we can use with our older NMR software?

Thanks,

Robert