Setting Period Change Settings "During Switch Plan" set to "Denied" in
Home> System> Settings> Customer CP
makes switching plan period impossible not only from Customer's Control Panel, but from Provider's Control Panel as well.
Issue will be corrected in PBA-E 5.5.2. Until update release please set the option to "Allowed" (or change it to "Allowed" temporary when you need to switch service plan period on one of the subscriptions).