OP states he has in the past been able to access and change his reservation after it was booked by the TA:
Originally Posted by
goatee87
In the past when I have purchased tickets through this travel agent, it appears immediately in my mileageplus account. I have also been able to make changes to the reservation as I need by paying the change fee through the United website.
So we're all just speculating here. OP is the only one who can find out for sure (either by checking with his TA or trying to make a change w/in 24 hrs).