Personally, I'd get a bottle of distilled water and just top it off.
Clogged heater core scares aside... it's supposed to be "lifetime" coolant, and we live in Florida where the demands on coolant/anti-freeze are minimal. If you still have the car when it hits the 10 year mark, maybe change it then!
When I changed my engine, my old coolant was clean and blue. I just drained what I could of the old coolant... and dumped it right back into the new engine. Topped off with a half-quart of distilled water.
If you did want to clean out the reservoir, a couple bolts would have it out and you could rinse it easily any way you want.
And I don't care what anybody says about "compatibility", if I was going to put anything other than the original Mitsu coolant in there, I'd give the whole system a serious flush (don't forget to turn the heat up to maximum with the car fully warmed up to get full flow through the heater core) so that you're at zero risk for mixing coolant types.
But, really... I would (and did) let it ride.
You wanna change something that most people don't think about? Change your brake fluid!