Propylene anti-freeze is going to be much more expensive since that particular compound is used in many other applications. It's also commonly used in medicines, and it's similarity to ethylene glycol is what inspired a Chinese drug manufacturer to produce some medicines using cheaper ethylene glycol. This resulted in deaths of over 100 people in Peru before someone finally figured it out.
If you're worried about it, then open up the popcock, drain half a gallon or so of coolant, and add straight coolant- not mix- to replace it.
I love my neighborhood puppies, but I've never used non-toxic anti-freeze. Whenever I have to do a job on one of the vehicles that will result in spilled coolant, I make sure once I've spilled it that I use a hose to wash it away and keep the hose on until I can't see green anywhere.
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