Why can't you use your old profile?
It isn't necessarily quite as easy as that. I can't remember the specifics, but I couldn't use my old Xbox Live profile when signing up for Live on the 360.
I'd been using the same gamertag as my old Xbox for my 360 offline profile at first (SomaticSense), with the intention of renewing my Live account (which had been expired for about a year or so) when some worthwhile online games were released. The problem when eventually signing up was that the old Live signup was done with an old credit card, which I'd since cut and thrown away a couple of years previous, so I obviously didn't have the exact - to the unit (which MS demands)- details of when I first signed up so that I could renew.
As such I had to create a new profile from scratch, purely because I couldn't use my current gamertag online because 'someone else' already had it (which was me, the stupid MS idiots....). Now the fact that this meant I was starting at 0 gamerpoints didn't annoy me, but it was the fact that no game saves are transferred either. You're forced to start from scratch, which is a complete pain in the arse when you have a large games library.
This, plus the stupid thing of downloaded content not being related to the HDD you downloaded them onto, but the 360 (which is a big problem when your 360 RODD's and you buy a replacement 360) makes me pissed off at the complete fuck up MS have made over the 'user-friendliness' of the console.