Seeting another planet as homeworld

I don't think you can do that with the latest version right now.

I remember asking at one point in time to have the civ capital stop moving around every time you conquored their homeworld (it made it difficult to capture the planet improvement). This was shortly after a patch was released that caused the civ capital to move to another planet (if possible) after a successful invasion. This effectively made it impossible for you to capture the civ capital unless you managed to completely destroyed another civ (or have them surrender to you).

Not being able to assign a new homeworld appears to be a by product of that fix. It only seems to be a problem for the Drengin who's super ability has their super dominator corvettes show up on their homeworld's door step.