Question regarding passport renewal

[moon]

What if the passport is renewed after getting a visitor visa? The visa will be stamped on the old passport which becomes void I guess. My concern is that my parents have an appt in the end of Aug, and my father's passport expires April 2006. If they plan to come next February, then my father has to get his passport renewed, as there has to be a 6 month validity after entering US. Is that ok or is it a good idea to get the passport renewed now and take another appt?
Thanks in advance for the help. And good job everyone.

[immihelp]

Get passport renewed.

If visa is in old passport, he has to carry both passports together.