In Pidgin, I can't edit XMPP account details after saving them

Why?

Try disabling this account before editing.

Correct, if the account is online or enabled and offline, you can’t change the settings.

Protocol, Username, Domain, and Resource are now no longer grayed out and can be modified.

Is that a question, or are you summarising?

I was adding additional context as it wasn’t clear from the screen shots alone what you were trying to portray.