At the risk of asking a really dumb question, is there any dire consequence if you do not get all of the old coolant out?

hi- I've done this task several time. The best is to use a bucket that fits unser the lower hose connection. Leave it there for a few hours if you can. When you flush the system use good clean water- the trouble with hose water is that the minerals attach to the metal in the block and radiator so use filtered water if possible.
I kept the water in the engine for a while as I drove around town. It cleanses the acid residue from the flush out of the block.
Place the used anti-freeze in the original container if U still have it- otherwise in one or two of the bottled water containers.
You will need about 7 liters of antifreeze 50/50 using distilled water. My experience with some anti-freeze compounds is that they go "sour" after a while and algae starts to build up in the system. so use a good antifreeze compound.
This is good info:http://www.matthewsvolvosite.com/anti-freeze-brand.html
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89-740 GLE;91-940 Sedan ;98-V90 Wagon ___ all running well

Well if your putting in a new heater core, you might as well make it last as long as possible.
Use A gallon of G-05 and a gallon of "distilled" water.

The specs say 10 quarts but I never seem to get that much out. I suspect normally there is extra left in the heater core and other places that may not drain well.

If changing / flushing: I drain, fill with water, run for a while with water, drain, add 5 qts Zerex, then fill the rest with water.

By the way if you want stock up on zerex G05, Zoro tools has it for $12.59. Free shipping for $75 and plus google a code for $15 off.
http://www.zoro.com/g/00039954/k-G2079122/
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Paul NW Indiana '89 744 Turbo 175K/ '90 745 turbo 145K

8 quarts is about what I get out with a simple draining. That makes it easy to get a 50/50 mix by simply adding a gallon of antifreeze and a gallon of water.

To get as much as possible out of the system I use the block drain and then loosen the lower radiator hose at the waterpump. There is no way around some spillage but with the lower hose still connected to the radiator you have some control over directing the flow as you lower the hose.

Several buckets of water to flush the area of any spill will dilute any reasonable spillage enough not to be a danger to peoples pets licking up undiluted coolant.

Now that the weather is moderate (at least in my part of the world) I would simple fill the system with water and drive the car. Remember to move the heater control to heat to flush the core. At the end of the day drain the system via the block drain. Refill the system and drive for a day and once again drain the system.

A gallon of G-05 and a gallon of water should get you close to a 50/50 mix of coolant in the system.

Randy