Is it a problem to change the IP of a qmail server?

By default qmail works and uses the primary IP address of your server so changing the IP of your server will not affect the workings of qmail.

Obviously, if you have made custom settings in tcpserver you will have to change them too.

Remember, if you change your servers IP address you may also have to change the MX records of your domain to reflect that change…otherwise people will have a problem sending email to you!