I guess Microsoft's main concern is that the RC is Windows Ultimate, and they didn't want people upgrading from Ultimate to a non-Ultimate version for various reasons.
Google will help you find a step-by-step guide on upgrading from the RC, but basically, copy the installation DVD to HD, go down the "sources" folder until you find cversion.ini, and change "MinClient=7233.0" to "MinClient=7000.0". There are more complex ways of upgrading to a non-Ultimate product.