My HVAC is a few years old and I have the old-type thermostat with mercury in it. I was considering replacing it with a digital one. Is this an easy "do-it-yourself" project? That may seem like a "duh" question, but I know a lot of people who talk about getting a professional to replace their thermostat, so I wondered if there was something more complicated than matching wires like when installing a ceiling fan. Is it simple, or are there some special tips i need to know if I plan to do it myself? Thanks.