If you want to change the server name on a Windows mobile phone (e.g. Motorola Q), the only way to do this is to delete the old setting first. Otherwise, the username and domain fields are not editable.
To delete the setttings,
1. Click on the ActiveSync icon and choose ‘Menu’
2. Go to ‘Options’
3. Highlight the ‘Exchange Server’ and press the the Menu button for the option to delete