If it were mine, used hard, and I had the materials on hand, I would do it as soon as it's convenient.
A drain plug makes it so easy there's no reason not to at least drain and fill, even if you don't install the filter at the same time. I changed to Mobil 1 in my easily driven Silverado around 36,000, and the original oil was sparkly. In my Vibe (bought used), it looked like motor oil with glitter mixed in at 28,000. If the pans didn't have drain plugs, both would have been a lot worse by the time I got around to dropping the pans by the old ATF bath method.
You have to disconnect or cut cooler lines to install the Magnefine, which is half the work of the flush anyway. You might check the total capacity vs. drain capacity, though. If a drain gets out most of the fluid, you could save the mess of a flush and just do a couple drains together followed by a regular change interval.