I don't understand 'why' to show a complete list of domain registrars while editing 'domains' in 'customer details'.
Why not just list the 'actived' registrars? Or just don't show that information, because this already setup in 'domain pricing -> autoreg'.
If this is a way to change between domain registrars, but I think this can receive a simple modification. Just show registrars configured. Create new administration roles for this, like:
1- Show only registrar select;
2- Show and allow change registrar select on save button;