Change it now, Bully. Get some of the official Toyota "pink" 50-50 pre-mix. Don't bother with the full-strength red, as it's too much hassle.
It's really easy to flush and replace the coolant on these cars. First, turn the heater to hot, and if you can, park the car with the front slightly downhill. Then, loosen the block drain bolt on the petcock (down to the left of the exhaust manifold while standing in front of the vehicle), loosen the radiator drain (lower right), and pull the fill cap.
I used a low-profile mop bucket to catch as much coolant as I could. Be patient, because the draining will take a while (the block drain is tiny). When no more is coming out, close the petcock and radiator drain and re-fill with the pink. Now you're ready to flush.
Note: In the OEM service manual, Toyota recommends NOT flushing with distilled water -- use only the 50-50 mix.
Run the car until warm, let it cool a bit, and then drain again. Refill with the pink with the cap off, get the engine warm, and then top-off. You're done!
Don't forget to remove and flush your radiator overflow tank. Fill it about 4/5ths of the way with the pink so that as the engine cools after the final fill, the level will end up just about right.
You might hear some gurgling in the heater core for the first few times you drive the car, but with each cool-down, more coolant will be drawn back into the system, and eventually the gurgling will stop. There is no bleed-valve on these engines, so just keep an eye on the cooland reservior, and add more as needed.
Two gallons of the pink'll do ya fine, including the flush.