The Tig torch Rotary Dial is limited to its rotation from low to high.
Just set the panel Switch to 4T mode and depress trigger one time and then dial temp up on torch as the amps are required or lower as needed. Keeping in mind of course that you have no set limiton the old 200DX

I have had some enquiry,s as to the feasability of retrofitting the new boards to the old models

For the new model 200DX the setting is completely different to the old . The new model also has pre flow added and to adapt the older version would mean having to not only change out the complete top board but it would need to have a Daughter board added for preflow.
I will discuss this with Engineering to see if it is a viable option to offer some type of kit at a somewhat reasonable price point but i am not overly confident that it will be done in a cost effective manner.