I am pretty sure you will get to use the new expansions on your old account. Why don't you just get the expansions online? I mean if you add it up, you probably save more than if you rebuy the entire game again.
As in regards to the Original sins, well, too bad on that front. I doubt that they will give you a refund for your old game. I know this from Steam and valve, more than a year ago I purchased Portal, and just recently I just purchased the Orange Box, which has Portal and valve just entirely ignored the fact I have Portal and won't give me a partial refund for the game.
