We have an older home that we are renting. It has an outdated, unreliable mercury thermostat and we want to upgrade to a digital thermostat. I checked and all the wires are there as we have the heat and a/c. Is it OK to do that?

I can see no reason why you cannot change your thermostat to upgrade it, after all it is only a measuring device... to put it simply... and doesn't interfere with the workings of your heating system

Make sure the owner approves of this. That can be a problem with modifications and usually when modifications are made they have to stay with the home when you leave.