Depends on where your license is located.
If on a server, you don't need to do anything, except load MathCad and configure the new computer to read the existing license.
If the license is local, you will need to use the license tools to transfer the license to the new computer's IP address, then install MathCad.
1. Use "Reconfigure Software Licenses" option from a PTC's site: http://www.ptc.com/appserver/lm/rsl/reconfig.jsp
2. Try to contact with PTC tech support by using "Mathcad Support Center": http://www.ptc.com/support/mathcad_supportCenter.htm