If it's still full of gas, then legally you're supposed to get it vacuumed down to remove all the gas. It's illegal to just vent it to atmosphere.

You need to take the aux belt off, but IIRC you can get the compressor out from underneath (on a RHD 3.0 anyway). Just remove the under-tray, and 4 bolts fix the compressor to the mounting bracket. The A/C pipes attach to the top of it, and getting to the securing bolt for these was the worst bit IIRC.
Once off, turn the compressor over by hand and empty out the old oil into a container. If there are any 'bits' in the oil then you'll probably have to flush the whole system out. These bits are probably metal, or oxides thereof, and if you just put the new compressor on without flushing you'll wreck it fairly quickly.
Put some PAG/PAO oil in the compressor before you fit it - at least as much as you drain out of the old one and anything up to about 100mL. Getting the A/C pipes back on the top are a bit of a game, and there are two rubber/nitrile/HBNR O rings that have to be got in the correct place - which you can't see.