If I'm not mistaken, a Vette Trans (4-L60) will hold about 13 quarts of Trans fluid. Just dropping the pan and replacing the filter and gasket will only remove about 5 quarts. The other 8 or so quarts are left sitting in the torque converter. So when you light up the engine, the pump in the forward Trans cover just pumped out 8 quarts of old oil into the pan. Now you have a mixture of both new and old ATF by the time you check and top off the fluid level. The only way to pump out all the fluid is to take it to a specialist who has the machine to capture all the oil and at the same time replaces the Trans with fresh ATF fluid.
The other old shade tree way, is to remove the cooler line from the radiator, over fill the transmission with oil, and watch the new color come out of the cooler line.
Use a little caution if you plan to do this. Don't pump the Trans dry. Just stop the engine, add more fluid and keep looking for clean ATF.
Why question when the fluid was change? Just dump it.
I don't know where this.... ( "if you don't change your fluid regularly then when you change it after 60+ k in miles you have a good chance on loosing the trans in a matter of days." )......came from? If you have a bad transmission, no matter what you do to it fluid wise, it will still be bad. Just changing the ATF will not destroy a "good working transmission" in days, no matter what the fluid condition is in. And this is oil that is just "overly dirty" and not contaminated....i.e. = friction plate burnt.
I'm sorry, but I am hearing an old wives tale about this, and question the validity of simply changing old Trans oil will render it useless in days.
My thinking is that fresh, thinner oil, will now have less tolerances between the friction and steel plates. Thus making the two plates lock closer together. A thicker oil between the plates, would cause slippage, because there is no "bite" between plates. This is how I see the difference between old and new oils in the Trans. If anything, the Trans will shift better with fresh oil.