Had issues aswell with resigning a save, so i'll explain you how i do it.
- Decrypt the same with Bruteforce
- Open the PARAM.SFO with HxD
- Search your USERID on the right (You can't miss it, there's 2 in the whole save, one around the start and one around the end)
- Select it and replace it with your new one (do it for both)
- Save, and re-encrypt the save.
- On Bruteforce, hit the refresh button and you should see the USERID has changed to the new one.
Now, the save is resigned on your account, but Online is still not working.
- Load your new save on the PS3.
- Start up GT6HE
- Wait for it to load, and then look for the PSN Name (in the Main tab)
- Put your new PSN ID in it, hit the "save button" then do something to trigger the save ingame. (Buy a car, or just do a manual save)
- Restart your game and all should be fixed.
Enjoy. If you need help, i'll try to explain with screenies, should be easier.