After viewing a few videos on how to remove the spray arm and filter below it, removing and replacing the rotted coupler (part 450, attaching the filter to the spray arm base) was easy.
Removing the spray arm required reaching around with a screwdriver to pop a lever on the back side of the spray arm to remove it, then unscrewing the filter and moving the lever the other way to wiggle it out. Then I popped off the rotten coupler, slid on the new one, and replaced the filter and spray arm.
I replaced the silverware basket, too, since the one on our 7 year old washer was cracked.