It's possible that the key you will receive will be an OEM key rather than a retail key, so it may not work.
However, you should be able use the installation media they gave you to reinstall over your existing installation and use the key that was provided.
There is a well known trick that you can use an upgrade license by first installing the OS without any product key, and then running the upgrade and using the upgrade key
So, in your position, I'll assume you're running the 30 day trial. At the end of your 30 days you can reset the activation clock for another 30 days. Then, when you get your copy in the mail, you should be able to pop in the disc, click on Upgrade, and install it right over your existing copy.
If you have an issue you can try changing the cversion.ini (You shouldn't need to).