Navigate to the mmb file in windows explorer, right click on the file and choose properties then deselect Read-only.
If the read-only option is greyed out click on the security tab, click the edit button which opens a second dialog. If the new owner is listed at the top, click the owner and then in the bottom check the full control allow option.
If the new owner is not listed, click on add then avanced. In the new dialog click Find Now and select the new owner from the now populated list at the bottom and click ok. Keep clicking OK on all the dialog boxes and your new treasurer should have full read / write access to the file. You will likely need administrator rights to complete this task.