That is personally exactly what I did with iGuidance 4.0. I purchased the upgrade, and installed it on my desktop for testing, then on the CarPC. I changed the motherboard in the CarPC, and never "unregistered" that PC before tearing it down, and they would do nothing about it when I called them. I "patched" 4.0, and was able to reinstall it on my new CarPC. I am still using only 2 copies, as their agreement states.
On another note, I use both in the car as Garmin is far superior for searching for say "The closest Target". Hell, I guess I cant even say its superior to that aspect of iG, as IG has absolutely no way to find the closest anything by name! One of my HUGE gripes with them! I have emailed them, shown then that ALL other GPS software has that ability with a nice chart I put together, pointed them to complain forum posts and threads based on that oversight ect ect... They said it would be in the next release... It was not in 4.0 as they said, Now iG 2009 is released, and they still have not included it.
Ok, back to my train of thought
I use Garmin and IG in the car. I use the Garmin bundled package with its GPS puck, as well as a BU-353 for iG. I have never had a conflict between the 2 GPS receivers, and am able to split the BU-#%# with xPort, giving me a Com port for Ride Runner, iGuidance and Garmin all at the same time.