I'm in the middle of changing the antifreeze in my Yanmar 2GM20FC engines and am
curious if I should change type.

I've been using Ethylene Glycol, following the advice of a Yanmar mechanic who
said using Propylene Glycol would void my warranty. I've never been able to
verify that, and I'm now long out of warranty so I'm wondering if its time to
switch to the more environmentally friendly Propylene. I also think that using
the easy to dispose propylene would encourage me to change more frequently.

Secondly, if I do change, do I have to flush out all of the old, or is it
sufficient to just drain it?

TIA, Jeff