Definitely not hard. You can look HERE and HERE for some general guidelines. You have a number of zone wires and one common. When you disconnect your old controller wires mark each one by the zone 1, 2, etc they come from. This is only for your convenience. They can be assigned to any zone but you probably already know each zone by the numbers. Really all you do is disconnect wires, remove old box, install new, reconnect wires and program.
The new digitals offer a number of features you don't have. You have a 12 zone controller. If you stay in the Rainbird line you can go with the ESP line with 3 expansion modules, assuming it's mounted indoors. Or you can go with the Hunter Pro-C line. Or any of a bunch.