SOLVED--on POP type account
I had the same problem--I could not edit the Incoming Mail Server field (it was grayed out).
The solution for me ended up being in the Advanced tab.
Open Mail, Go to Preferences, then click Accounts in top nav.
Then in the left column, click on the account for which you need to change the Incoming Server info.
Click on Advanced tab and uncheck the first box "Automatically direct and maintain account settings"
***IMPORTANT NOTE
Once I UNCHECKED this box and saved the changes (to save the change, you need to click other account from left nav or close preferences window, only then do you get the prompt to save the unchecked box change).
Then go back to Account Information tab at top.
Now the Incoming Mail Server field should be editable.
Overly complicated and unintuitive, but now I am able to receive mail.
Hope this help someone.