Xbox live profile problem

So i created a offline account on my new xbox, i put my Nick/Gamertag (which i used my whole life in every console i owned), i started playing, and being happy all the way,

The problem came when i tried to access Xbox Live, and i found out that my user name was already being used by another 360 owner, and i was forced to create another account, so my problem is: i already have all my achievements on my gamertag, but i cannot connect with MY gamertag to download stuff to my 360, and if i want to play games that need downloads, i have to use my other gamertag to connect, but then i cannot use the downloaded data to apply it to my offline gamertag.

Anyone knows how to access or copy that data to my profile???

Any idea will do it, (by the way i don't care about online playing, i care about downloadable content)

You shouldnt need to create a new account and it should allow you to change your name until it becomes unique.

You must be missing something somewhere.

No it doesn't, i downloaded the NBA 2k9 roster update and my offline profile cannot access the data downloaded with the other profile.Cexr