If a feed's URL changes, you can have its LiveJournal syndicated account updated by e-mailing email@example.com indicating the LiveJournal username of the syndicated account in question, and the new URL of the source feed.
If a feed's URL changes, and the new URL has already been used to create a new LiveJournal syndicated account, you can have the two accounts merged. To have this done, e-mail firstname.lastname@example.org indicating the LiveJournal usernames of both feeds and the URL of the newer, working feed, making clear which LiveJournal syndicated account should be kept and which should be turned into the redirect.
This process not only updates the information of syndicated account X, but it merges the readerships of accounts X and Y and turns account Y into a redirect for account X. By way of an example, the situation I described in my previous post has been resolved so that downhillbattle is now the correct feed, and dhb_music now redirects to downhillbattle. Anyone who had dhb_music friended now has downhillbattle friended instead; this has increased the readership from 36 to 39. (I couldn't even type <lj user="dhb_music"> in writing this example because its redirect turns it into downhillbattle. You'll notice the change in my previous post.) Fun stuff.
Questions? (Other than who the Special People™ are, because that's a closely-guarded secret.)