I have a question about this too.
I'm new here. I got a new thermostat put in yesterday (2001 Celica, 150K miles, no modifications) because the old one was sticking. I brought a container of the red coolant in for the mechanic to put in, but when I picked up the car today, he said that wasn't the right type and that he put green in instead.
I asked him why green because I had read that the red was the correct one for my car, and he insisted that green was, without giving me an explanation (I pressed him for one, but there was a bit of a language barrier between us).
I went online again wondering if maybe he did that because it was an issue of my car always having had green coolant before and that the two couldn't be mixed. I read somewhere that when it's flushed, sometimes 15% or more doesn't come out, and that the green coolant and the red coolant shouldn't be mixed because it creates an acid that eats away at the parts.
But it sounds like the green coolant alone eats away at the parts as well.
Does anyone know which scenario is worse? Sticking with the green, or trying to flush out all of the green and then using red? If I should do the flush and get the red put in, how do I communicate efficiently to a mechanic to do that and not do something else entirely? I wish I had the means to do this myself and not rely on a mechanic, but I don't.
I've started trying to set aside money for a new (used) car, because this is probably a losing battle...