FAQ 25 (renaming, under Accounts) mentions how to rename to an existing account under your control, but it's something that's easily skimmed over, I think in part because the main part of how to do it is the second half of a sentence, joined by a semicolon, and so not the most obvious positioning. I suggest the following change (italics are old content):
It is possible to rename any personal or community journal to an unused username. Additionally, personal journals can be renamed to the username of another personal journal under your control; both journals will need to have the same password and the same email address. (This does not combine the contents of the two journals.) If you are renaming to an existing account, both journals will need to have the same password and the same validated email address.