In my company, we changed our email hosting provider.
The username used to send and receive mail is not the same.
With Outlook or Thunderbird this was no problem. We just had to change the username in the account settings.
But with Windows phone, changing the username displays a message saying the current account will be deleted and recreated.
I tested this and can confirm this: every locally stored emails are deleted (interesting: iPhones have exactly the same problem.)
A workaround is to create a second account. But the first account keeps complaining about the fact that the server can't be reached, or something similar (I set the check frequency to manual).
And this is ugly.
I search for a way to change some/all email settings without the account being deleted.