I'd like to suggest putting the lines/information about 'existing comments/comm-entries will always show your new username, regardles of your redirection choice' closer to where it actually talks about the redirection setting.
Currently, the information is given in the last paragraph of the first section, just before the technical section of 'how do renames work':
"If you want to hide your journal from someone who has found your username, the rename feature is probably not what you want. Comments and community entries that you have written will display your new username; people may also be able to find you based on your community memberships, interests, friends list, and so on. Instead, you should consider deleting your entire journal or making it Friends-Only or Private."
A user who answers 'no' mentally to the question of wanting to hide the journal will most likely skim or skip the rest of that paragraph -- they read the first few words and go "that's not what I want to do, so I don't care about what it says there."
I'd like to therefore suggest that this info be repeated (rather than 'moved', as I think it's needed in that paragraph above just as much) in the paragraph where the redirection option is specifically talked about. I've bolded my suggested wording, but it can be tweaked/moved as necessary:
"You have two options for what will happen when someone tries to access the old username. The old username can automatically redirect users to the new username – using this option is encouraged, so links to posts in your old journal still work. The User Info page and journal page for your old username will automatically forward to the User Info page and journal page for your new username, and any LiveJournal user tags will automatically display the new username instead of the old. This redirection will remain in place as long as your account exists. Alternately, the old username can be marked as deleted, so anyone who visits the journal will receive an error message. The old username will remain unavailable for at least 30 days. However, note that any existing comments or community entries you've made elsewhere will automatically display the new username, even if you choose to have the old username deleted. There is no way to change this behavior. The option to delete the old username only affects what people will see when they search for or visit your old username directly."
Maybe even split that off into its own paragraph, as it's actually a fairly important concept that people get 'caught' on a lot.
Anyway. Just my thoughts on it. Do as you will. ;)
