let's say my Gmail is firstname.lastname@example.org. Sometimes I want to send mail as John, and sometimes as John Smith from the same address. Is there a way to do this without having to change the "Send mail as" settings everytime?
Why change the "Send mail as" settings everytime ?
Just add yourself to the list
The disadvantage here will be the appearance of your email address.
Now you can scroll between appearances within the "Compose Mail" section.
I think Premium Gmail supports email aliases. I think an email alias will address your problem. I dont think you can create email alias with the classic gmail account. Correct me if I am wrong...