How can I replace an air conditioning thermostat?
It should be possible in theory to use a generic timer switch to electrically disconnect the 'stat from the aircon during certain time periods so the 'stat cannot call for cooling. Whether it's inexpensive or easy (or attractive) depends on the details of the current installation and the style of timer switches available.

A better solution would be to replace the 'stats with a more sophisticated controller that has timer functions. This solution is unlikely to be inexpensive, but shouldn't be prohibitively expensive either. It is typically easy to do this, disconnect the old 'stat from the wiring and attach the wiring to the new 'stat. Once again, the ease depends on the details of the current installation and the replacement 'stat.