It worked! My threads all now show up in the same registered account just fine. --- My apologies for bugging you earlier: problem resolved.

Thanks for your clear advice Michael. I followed your steps and filled out the form. Then I replied to two emails: one for each account which I had created. I received a statement saying that the unregistered one would be removed and my unregistered answer transferred. But at this moment, over 13 hours later, no change has actually occurred: the previous question is still attributed to my unregistered user. Does the change take a long time to go through?