I want the process variable type to be updated with the current version of CDT f

This post should help -

forum.appian.com/.../e-147707

I guess the above mentioned feed is all about updating the CDT values and the process upgrade to latest version as of Process Model. My requirement is to update the version of CDT to current version of CDT in active instances.

If I understand your requirement correctly, I think the second reply should help you with this:

"For example, if the target model has a PV (`pv!person`) of type Person^2 and the source model has the same PV but of type Person, the process will not upgrade because the CDT types are different."

Therefore a possible solution is to add add a process variable of the new type leaving the existing one pointing to version ^X of the CDT then remap nodes to point to the new variable which will be indeed of the updated verison.

For backwards compatibility in processes Appian doesn't let you update Person^2 to Person since there can be places where the mappings to fields won't match.

So the only way to add a new field to an existing CDT on an active process is add a new CDT to the process and map to the new CDT and then upgrade all the old processes? Do I understand this correctly?

Or bypass the process model altogether and use query rules from SAIL to the entity directly ...