Provider cannot change Service Template for a subscription on standalone POA installation - after pressing the 'Change Service Template' button in a subscription properties the drop-down list of the avaliable Service Templates is not shown.
POA only shows single Service Template like on the screenshot below and it is not the Service Template Provider want to switch the subscription to:
The subscription in question belongs to a reseller, while the desired target Service Template belongs to the Provider.
Create the desired Service Template under the corresponding reseller account (owner of the customer account in question) at Operations Director > Reseller Manager > Resellers > select reseller > Subscriptions > Service Templates and try to change Service Template again.
Make sure that the Service Template is active if it is absent in the list of available templates.
The same actions may be done in the POA Reseller Control Panel.