I know that there are different companies who supply the microchips.  The
ones my vet uses are inserted with only 20g needles and the chip itself is
microscopic and can't be felt.  The number of the microchip is stored with
the company who supplies the microchips, but unless you pay an extra amount
($25-$50) it is the veterinarians name/number/address that is stored in the
computer.  Therefore if you move over the years, you would need to notify
your vet rather than to notify the microchip company.
 
A side note: we're also finding that most microchips (inserted in the scruff
of the neck of cats, dogs, rabbits and ferrets) travel down under the skin
to the belly area.  (Important to let the people who are scanning stray
animals know that this occurs.)
 
Sandi
Best Little Rabbit, Rodent & Ferret House
Seattle, WA
[Posted in FML issue 1998]