Based on the info here, I changed my email address. Everything looks good. When I changed my email address under My Account, it sent an email off to the new email address. I had to click on a link in that to finalize the change. The email also has a little blurb in it saying:
"If you are using the same email address for your Stardock.net Account as you have used to purchase Stardock products then there is a good chance that when you change your email that you will also be changing your Registered email address. This is ok. Just remember that when you look up product updates that you may need to use this new email."
By that, I assumed they're talking about Stardock Central. When I started that up, it told me my email address wasn't correct. I went to Tools | Edit Stardock.net Account and changed the email address there and connected fine.
For the forum, I just logged in with the new email address and all was well. In both the forum and Stardock Central, all my registered games appear to be present. We'll see what happens when Twilight of the Arnor is released (I've pre-ordered it).