Oh, they're technically safe to mix so if you've had it in there for a while I wouldn't sweat it. Nonetheless, changing the fluid completely isn't a bad idea.... It's probably never been done before.
I would use Dex VI for the lower viscosity, higher shear resistance and lower friction coefficient. I'd change it out a second time after a month or two because all the fresh detergent it likely to loosen up a fair amount of crap.