Unfortunately, that rather depends upon what the problem is with any particular worksheet. I think you'll have to post a few examples to see how bad the 'damage' is. It might be trivial or it might require major structural changes.
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Mathsoft made a significant change to the way Mathcad works in Version 12, by introducing something called static type checking, informally known by some as Static Unit Checking (SUC). SUC was hard-wired into Mathcad's operation and could not be switched off. The intent of the change was reasonable from a particular point of view - to prevent people from making units and some other mistakes by detecting problems before at design time rather than let a worksheet run for a long time before coming up with some cryptic error message. A major factor in the change was the complaints from new users about the difficulties in tracing errors under the then existing dynamic type checking.
However, it wasn't very long before the more experienced users found that SUC truly Sucked. In implementing SUC, the designers decided to remove any potential for a type (or unit) mismatch. For example, I had several functions that analyzed data according to a function that I passed as part of a vector argument (very useful for generating tables or statistical analysis). However, Mathcad couldn't guarantee that I would always pass,say, a 2-argument function f(a,b) as opposed to a 3-argument f(a,b,c), and it couldn't guarantee that I would always pass a function in the input vector... so it prevented the use altogether. In short, it acts Nannyware by stopping the user doing things that could hurt them at run time. It really was a case of Health and Safety Gone Mad! Mathcad 12 also introduced some other significant changes to the way Mathcad operated that were not well received by experienced users and not because It Was New. It also closed a number of 'backdoor' symbolic features, which was annoying and put an arbitrary limit on recursion depth (again, to prevent the inexperienced from suffering the consequences of not writing their algorithms with adequate 'stop' checks). In short, SUC ended up stopping user from Hurting Themselves by the equivalent of taking away real bullets from soldiers in the field to prevent Friendly Fire incidents (OK, I exaggerate, more like chopping all the trees down to prevent children falling out of them in the playground).
You're lucky you've got Mathcad 13, whilst Mathsoft stubbornly retained SUC, they did fix many of the other Problems with M12 and M13 .. M15 are more usable, albeit still hobbled by SUC. The new Mathcad Prime has removed SUC but introduced its own 'features' - however, it is a step in the right direction.