it's only a bitch in so far as you must remove the whole dash to get the heater box out.

why do you have to change it? cos it leaks?

if it doesn't leak, but doesn't give you hot air, there are a few reasons.

1) the cable is no longer moving the air divert flap to send the air thru the heater core - check that there is a change in sound of the air flow on max. blower setting and you should hear the flap come to a stop.

2) airlock in the coolant system stopping coolant flowing thru the core - are the 2 pipes going to the heater hot?

3) heater core blocked with shitty water/freezeaunty - remove both hoses and try to flush thru backwards (opposite flow direction to normal - ie. put hose pipe on the outlet of the heater that normally goes from the head to the heater). if it doesn't flow much and is all dirty, connect a length of hose pipe and pour some mr muscle drain cleaner in there and leave for a few hours. flush it out and repeat as necessary.

hth