Be careful with the wire colors. I've installed replacement programmable
thermostats in two houses where some genius spliced the wire up in the
attic, but didn't match up the colors.

wrote in
ps.com:

theromstats are low votage units, the AC and furnace will shut
themselves off as soon as you disconnect the thermostat. it may be
anywhere from 2 to 6 wires or more for complex systems. fortunately
there is a standard color system for the wires. unfortunately this
means the installer knew that. normaly the terminals are labled for
color, so you can just look and see what terminals are being used by
what wire and move the wires to the new thermostat.