Evolution migrates your mail box to a new format, maildir from mbox in Fedora 15. Always backup evolution using the in-built menu option File -> Backup evolution data. This generates a tar.gz file which you can use to restore evolution later. From Fedora 14, this should have been your backup approach from Evolution rather than saving the .local/evolution files. If the path changes, you lose your mailbox. I lost mine when moving from Fedora 13 to Fedora 14. Now I am much wiser.
You are asked to migrate your email box from mbox to maildir. This migration has to be done. When this activity is done, you will find 2 mailboxes for your account in Evolution. One with “On This Computer” and the other with “user@hostname”.
After your migration, go to Edit -> Preferences -> Account. You will find a protocol stating mbox. Remove that account after you verify everything works well after you migrated in your new evolution account.