quote Xbox.com

Q: I created an offline profile on my Xbox 360 system and played a bunch of games. I also created a gamer card on Xbox.com to reserve my gamertag until I'm ready to join Xbox Live. Now that I'm ready to join, how do I merge these two profiles?

A: You can do this, but be very careful—it's possible to do it in a way that wipes out all the games and achievements you've associated with your offline profile, and you probably don't want to do that!

If you've created an offline profile on your console and want to keep it, and you've also created a profile at Xbox.com, you essentially have two separate profiles, one on your console and one on Xbox Live. To get your offline profile online with the same gamertag, with all your achievements intact, you have to get rid of your placeholder Xbox Live profile and supplant it with your offline profile.

The tools for doing this are all on your Xbox 360 console; you just have to do some shuffling around to make it work.

To walk through the procedure step by step, see Merge an Offline Gamer Profile with an Xbox Live Gamer Card.