Meena -
Conversion issues depend on what version of MUMPS code you have and any special functions the old code might be using that don't have a direct replacement in Cache. Also, the old MUMPS code might be calling outside MUMPS to OS functions that a modern platform might not have.
If the original programmers were careful they may have used non-exotic MUMPS and the conversion will be easier.
The basic concept for conversion is to save routines/globals on some media (hopefully that your old/new platforms have in common), then move the routines/globals into a namespace on the new platform. Then start testing the code to see what might or might not work. Typically a programmer would build their own conversion tools customized to the conversion issues that came up.
Conversion can be non-trivial, again depending on the old code, but is usually feasible.