The short version of the problem:
1) Download and install demo, play it, love it.
2) Buy game, download and install SD Central, install Sins of a Solar Empire
3) Add/Remove Programs, Click "Remove" on Demo, and get the following message:
(paraphrased from memory)
"You cannot install the Demo when a Retail copy of Sins is installed. Please remove the Retail copy before installing the Demo".
Looks like a check was in there to assist in development/debugging, but here it's preventing REMOVAL of the Demo.
Considering you'd normally get the demo, then hopefully buy the game, this seems like it should have been caught already.
As an aside, the demo has a bug in the 1.5 hr timelimit - my second game in the demo went for 4 hours, unfettered. Not sure how I did it as I couldn't do it again the next day.