I follow this.

Backup

1. Right Click on the Inbox folder
2. Get the path to the folder.
3. Browse to the folder and Copy everything under it.

Keep that folder somewhere at safe place

Restore.

Perform Step 1 and 2.
Paste the folder which you kept at some other place.

Now I dont think you will need to restore for the upgrade but if something goes wrong then it will be helpful.