QUOTE(lmellor @ Mar 6 2010, 07:25 PM)

I won't lie, I managed to do one, then I had a few drinks and tried again and couldn't do it.
Tomorrow I intend to attempt to do the other one again, I believe that what I actually did was to extract the Account file from the Profile of an offline that I had created, changed the gamertag of that Account file to the name of my own, injected that Account file into my profile, rehashed and signed everything then over-wrote my profile.
I'll know better tomorrow given some sleep

Haha, that's what I just did after having a brainstorm moment. It still shows as Live in some places but never tries to sign in, so that's good enough for me.
Steps;
MAKE BACKUPS BEFORE ANY SAVES. I won't tell you when that is, use your brain.
Copy your profile (hdd1/Content/E<whatever>/FFFE07D1/00010000/E<whatever>) to your PC.
Create another profile, offline non-Live (Or use another profile, doesn't matter) and copy it too.
Open Modio.
Use the drop-down to find the Profile editor.
Open the OFFLINE Profile, Extract the account, tell it where to save it and name it somethning you'll remember. Click Yes to load it, Decrypt the account, rename the Gamertag to match yours. Close it.
Open the ONLINE/to-be-edited profile. Inject the other account - the one you saved in the step above - into it, using the Inject button. Close it.
You may or may not need to resign/rehash the profile by drag-and-dropping the profile into Modio, and clicking Rehash and Resign. I do it anyways, it's good practice. Upload it, overwriting your old one - remember the backups? Hope you have them, just in case.
Done. YMMV.