Head to your discography, view the album containing the track you want to move, and click the Edit button beneath the title.
In the album editor, select the track and then click the "detach" link at the bottom of the track pane. Save the album.
Head to the discography, view the newly-free-floating track, and click Edit.
Check the "part of an album..." drop-down, select the new album (which must be published, not a draft, to appear).
When you’ve selected the right album, don’t forget to click "Add to Album"! Voilà.
How do I move a track from one album to another?
To move a track to a new album, open the original album's editor, select the track, click "detach," and save. Then, open the detached track's editor, select the newly published album from the dropdown menu, and click "Add to Album."