but don't buy the game right then when prompted, will I get them unlocked automatically if I buy the full game later, assuming I don't delete the demo first?

Or will I need to play those sections and re-earn them?

I've had a 360 since launch and I've never actually figured this out.

If you exit the game, yes you'll have to do it again to get the achievement/awards.

@nemt: It depends on the game I believe. Some games will offer the full game when you unlock an achievement and allow you to keep it. If you don't take the offer, you would have to earn the achievement again.

This how I bought Section 8: Prejudice. I played the demo and the cheevo for landing on a guy while free falling, so I had to buy it.

If you don't buy it right then, I'm pretty sure you always have to get it again.

If you don't buy it right then, I'm pretty sure you always have to get it again.