Every copy of Prime comes with a free license for Mathcad 15 (you buy the tail and get the dog for free, as someone here has called that).
The Prime3 license file (not the product code) is valid for Mathcad 15, too.
You can download Mathcad 15 install it and point it to the license file of Prime3.
http://www.ptc.com/product/mathcad/free-trial/thank-you
And MC14 and MC15 share the same file format - no conversion necessary.