In case you were wondering about installing the Lync release candidate now, then upgrading it to the RTM version:
There is no supported way for customers evaluating the public bits to do this.
Normally build-to-build upgrades for pre-release software are only supported for customers who are in one of Microsoft’s early adopter programs, like the Technology Adoption Program (TAP) that various product groups run. In the Exchange 2010 (and 2010 SP1) TAP, build-to-build upgrades were fully supported, and we spent a lot of time chasing down problems that occurred because of them. In that light, the Lync product team’s decision not to support upgrades from the RC to the RTM version for ordinary users is perfectly understandable, though disappointing. (Of course, Lync TAP customers are fully supported for build-to-build upgrades.)
Having said that, I am eagerly looking forward to hearing from people who install the RC now and then upgrade to the RTM version later. I’d love to hear what the pitfalls are.
(Edit: clarified that Lync TAP customers are supported for b2b upgrades, just not the rest of us. This puts Lync on equal footing with Exchange, SQL Server, and most other MS server applications.)
Paul's Down-Home Page 
I seem to recall that they did the same thing with the public beta of Windows 7.