FYI, in researching this I came across this article from MaineSail. It outlines some of the steps required - seems a bit of a pain to remove the heat exchanger to replace the collant, but unless somebody has any easier suggestions I guess that'll be the path I follow:
If changing antifreeze, the best thing to do, especially if your engine had or has "Death-Cool" in it, is to thoroughly flush it out, clean it, and start fresh.
To do this on a Westerbeke;
1- Remove t-stat and replace cover housing using a spare gasket
2- Remove HX (might as well clean and have pressure tested)
3- Purge old AF out of engine with pump into 5 gal bucket.
4- Use an acid resistant pump and circulate Rydlyme through engine block to clean cooling passages
4- Flush Rydlyme with water then fill bucket with distilled water and circulate through engine
5- Drain off what you can of the distilled, replace HX, t-stat and add antifreeze. The "Globals" are good. Death-Cool (AKA Dex-Cool) does NOT like all the dissimilar metals in marine engines and can create Dex-Sludge.
6- Purge air and test AF concentration with AF tester..
For air purging I replace the cheap "pet ****" on top of the t-stat housing with a 1/4 turn ball valve. I then add a barb and some clear hose to it. Run the hose well higher than the highest point in the cooling system. Fill the hose with AF using a transmission funnel, open valve and start engine. As the t-stat opens and closes AF will gravity feed into engine and no air will be left in the system. Air=bad. Once the system is no longer taking AF out of the clear tube close the valve and drain the hose back into the AF container..
Air Purging Valve: