Specify a password for the guest to change his nickname.

[User2]

Hello. I was looking at an old topic on my forum and decided to change the guest's nickname.
I wrote a new nickname in the "Name" field. The "Email" field is empty. I click Save, a message appears that you need to specify an email.
But this is a guest, he doesn't have an email. Why should I email him?
In general, there are no guests on my forum. It used to be a user, then I deleted it and it became a guest.

[Illori]

That form is to send the user an email. You can't change the guest name.

[Kindred]

As illori says, you can't change the name of a guest...  most especially,  you can't change the name on a deleted account (after the account is deleted,  it defaults to showing the user's original login name)

Direct database edit of the messages table is the only way to change the displayed name

[Aleksi "Lex" Kilpinen]

I don't believe 2.1 should ask for an email there. 2.0 used to though I think. What version is this exactly, and what theme are you using?
Also, the name can be edited in 2.1 no problem - Just tested this.

[User2]

Aleksi "Lex" Kilpinen, version 2.3.1, theme Default.

[Aleksi "Lex" Kilpinen]

I wonder, could that have something to do with it being a post of a deleted member instead of an actual guest post.
I was just testing with guest posts I had handy, changing the name wasn't an issue and I never saw the e-mail field at all.

EDIT: Yeah, that's probably it - If the post has already an email address associated to it, you can't remove it - You can change it though.

EDIT2: Try the setting "Do not show the email field for guests' posts" and see if you can then save the post without an email? Mind you though, this will completely remove the e-mail requirement from all guests. I believe this setting was the cause for the differing replies earlier as well.