If you have your PCStitch on a CD there's no problem, just install it on the new machine.
If it is a download, you were given an unlock code. You should still have that code available. Just download on the new machine and unlock it again.
If you have any problems, you should contact Mark using the Contact Us on the main PCStitch site for help.
One point - in any case, you should UNINSTALL from the old machine because you have one license. You do not want someone else using your "old" copy illegally.