my husband has a 2001 GMC Sierra and he has the same problem, however if you leave his on the setting you want then after some time it eventually will kick over to A/C or heat and vice versa. It's a pain in the ass though.
cold air to hot it will not change.? I have replaced the heater control panel on the dash with a new one but still will not work. When truck was still under warranty, it happened 2 times and the dealer replaced control panel twice. This is the third control panel, but this one did not fix the problem.
These work on vaccuum to change everything. Could be a vaccuum hose that got dislodged , or maybe left off during the installation of the new control panal. The lady's husband that answered the question may have the same problem, except just a leaky hose, so that it eventually is able to adjust itself. Check for a tube under the dash that is just sucking air.