It's possible to upgrade your existing Release Candidate installation of OCS 2007 to RTM. Alternatively you can do a clean installation (RTM bits here here and Office Communicator bits here). Should you go ahead with upgrade, here's a few tips you should keep in mind:

• Release Candidate upgrades to RTM only if you have the Volume License (VL) bits - and really, the bits, not just the license
• Evaluation (trial) license does not upgrade to MSDN RTM - at least, it's not tested
• Release Candidate does not upgrade to MSDN RTM

I find it almost always easier to start from a clean installation rather than do upgrades, but in certain scenarios it's often necessary to perform a direct in-place upgrade.