Used to deal with microchips (became the bane of my life making sure details were up to date for the cats in the rescue
)
For cats that are already chipped most chip companies will ask an owner to fill in a form to change owner details - they do charge an admin. fee. Identichip is £5 and PetLog £15 I believe.
Most won't charge a rescue to change details if the cat has been taken into the rescue and then adopted.
Overseas chip registration are treated the same as new chips inserted and a fee is payable.
Some companies operate an on-line system for owners with user name/password but think still ask for a form for change of owner/address details for security reasons.