The only way to make that work would be to keep something like M11 around. Personally, that's what I do anyway, since M11 runs faster for the types of calculations that I do.
M12 limited the oldest version compatibility to 2001i.
From everything I've seen on the subject, you don't really want to be using M12 anyway. M11 is the last good version unless there is something in M13 that you absolutely have to have.
I owm Mathcad 2000 and ,Mathcad 12 and I am mainly use 2000. However, just yesterday for a large simulation I have found that 12 is faster 10 times than 2000 for the simulation I am doing. So, 12 have somthing good thimg not just bad things.
Mathsoft concentrated on making some of the more complex functions run faster, at the expense of the simple stuff. Additionally, the static unit checking has also been a big time sink.