Hi Nicolas,
It's generally considered a security risk if billing and delivery addresses are different, if someone got hold of your credit card details they could use your card and address but have goods delivered to themselves at another address. However there are circumstances where having a different delivery address is necessary, we have customers who order from their business, at one address, but request goods be delivered to a studio they are using.
Also the billing address has to be the same as your bank account address when paying by card, otherwise the bank security checks will fail, so it shouldn't be possible to randomly change the billing address.
If a customer moves address then they edit their account details and change their registered address.
I'll try the code you suggest, if that limits the billing address to just the registered address it will do what I need.
Thanks,
Ian