I signed up on these boards with Twin Dragons as my primary account, but found that the acronym clashed with somebody else's, so I created Twin Agate Dragons as an alias and have been using it over Twin Dragons. I've come to be fond of my new handle as well as other people recognizing me with it.
What I'd like is for all my old posts with Twin Dragons to be merged with Twin Agate Dragons and have TAD as my primary account if at all possible. If it isn't I'll simply be happy with switching the two; Twin Dragons as an alias and Twin Agate Dragons as my primary.
In your account, under the alias' part, you can click "Make Default Alias." I don't know if it makes that your primary account or whatever, as I've never had the desire to switch my main alias, but I'm pretty sure it'll make it so you don't have to keep using the little drop box at the bottom of the message post to switch to Twin Agate Dragons.
Hey, I even just noticed, you can make the switch in the middle of posting by checking the "Set as Default Alias" box next to "Post As" drop box.
I'm quite sure that changing the poster for old posts is not something they offer, though. It would take time and effort and the it doesn't really offer that much in return.
That's a shame, I would opt to do this if it were possible.