Flying UA on about a quarter of my NA flights, I thought I'd fork out the $329 to get a year of Economy Plus with UA. That I did and lo and behold at YVR last week I was told by a UA agent that I would have to credit all of my miles to Mileage Plus! So I went back and reviewed the T&C for this subscription and there it was in black and white - all flights enabling E+ seating must accrue to a Mileage Plus account!
Lesson learned - case closed.
You should be commended for taking personal responsibility for an oversight, something sadly all too uncommon IMHO these days.