I use GoDaddy and they require me change my WhoIs record with in 5 business days.
I don't want to be a mean competitor, but what happens if they do not change the record?
Do your competitor a favor: Contact them and suggest that they may have a problem. You may find out that they don't, as the domain name registrant may actually have some continuing rights to the domain name until certain conditions are met - such as a "final payment".
[edited by: Webwork at 8:22 pm (utc) on Mar. 21, 2007]