The replacement unit is actually a lot closer to the standard designations, which are:
R = common live wire-- switch to G, Y, W etc. to make things happen.
X = neutral return wire (used for the lights on the thermostat)
G = starts the inside fan
Y = starts the outdoor unit compressor
O = sets the reversing valve for cooling
B = sets the reversing valve for heating
W / W2 = turns on electric heaters / emergency heaters
Go to the other end of the thermostat cable, which is inside the interior unit, and see what wire colors are connected to which terminal names.