You can switch between Windows and Mac without penalty. NXPowerLite Desktop registration keys are designed to work on both the Windows and Mac versions of the equivalent version number. So a version 7 key for NXPowerLite Desktop (Windows) will also unlock version 7 of NXPowerLite Desktop (Mac).
You don't need to let us know if you want to cross-grade. Simply download the version of the software that you have a licence for to your new machine and use your existing key to unlock it. All current and previous versions of NXPowerLite can be downloaded here:
All keys are also backwardly compatible, so each key can unlock the current version and all previous versions too.