What you had was likely a Technet subcription. It's basically a subscripition to "test" all Microsoft software for a period of time. Alot of people use these subcriptions as their permanent product key even though they're not intended for that use. If that's something you're interested in, keep in mind that you must buy a new subscription each year. This is cheaper that buying retail versions if you use a lot of Microsoft products.
Anyway, I don't think you'll be able to use a Home to Pro anytime upgrade key. I could be wrong though. Either way, there are some restrictions on the cheaper keys. The biggest difference between retail and oem is the fact that OEM keys don't come with product support and they are locked to the hardware you activate them on. In other words you can't call up Microsoft with questions and if you want to change your cpu or motherboard, you'll need to either buy a new product key or sweet talk Microsoft into allowing you to switch to the new hardware. They sometimes do this in the case of hardware failure, etc. If you're planning on sticking with the same hardware for a while I'd go with the OEM key, otherwise you might want to go with Retail.