First make sure you entered the new account information into GrabIt. Either by changing your username / password in the Server profile, or by importing the registry file provided with your new account information.
If you are sure you entered the new account information correctly, but there are still items stuck in the batch with the 502 error message, try downloading them with another server profile by following the following three steps:
- Switch to the Batch tab
- Right click on the item you are trying to download
- Select 'Download with other server'
This will open a new menu with all your server profiles. Choose one of the new server profile here and see if that works.