You don't mention my suggestion, so I assume you have rejected it as an option, but I will reiterate with an example, just in case I wasn't clear. You have a drop down field "Field A" with choices "Choice One", "Choice Two" and "Choice Three". Later, you decide to rename "Choice Two" as "Choice Four". If you duplicate "Field A", you will get a copy called "Field A copy". Rename the original "Field A" as "Field A old#1" and hide it. This way no one using the form from this point on can interact with the old field. Reanme the new "Field A copy" to "Field A". Edit the new "Field A", renaming "Choice Two" as "Choice Four". Since this is a new field, it won't have any data in it, so renaming the choice doesn't lose any historical information.
Of course, you now don't have all of the data in a single field, which causes other problems in creating reports. This is obviously quite far from an ideal solution and may be completely unusable for your situation.
By the way, I do agree with you that some kind of complete historical tracking of changes to a form, whether by the method you suggest or some other, is badly needed to make Machform more "enterprise friendly". However, the staff at Machform may not be interested in moving their product in that direction, so I wouldn't count on the idea that it will be added soon.