Can't find the option to change the subscription Service Plan in the "Power User View"
It is an expected behavior, Plesk Panel in Power User View does not use service plans for subscriptions.
This behavior is described in Plesk Administrator Guide:
To better meet user needs, Panel offers two interface views: Service Provider and Power User. These views define what panels you use, what tools are present in these panels, and how the tools are organized.
Service Provider view.
This view is convenient for Hosting Providers as it is intended for selling hosting services. It has all the features required to create and manage customer accounts, subscriptions, and service plans. This view includes the Server Administration Panel and the Control Panel. Service Provider is the only view that includes integration with Customer and Business Manager, the billing system for automatically invoicing and charging your customers. Even if you use an external billing system, but require an interface for your customers to control their hosted services, you should use this view.
Power User view.
This view is the best for power users - those who use Panel solely for personal needs, such as for maintaining a company portal or a mail server. In this view, both server administration and hosting services management take place in the Control Panel. As this view does not assume a reselling service to others is needed, it does not provide facilities for managing hosting plans, subscriptions, resellers, and customers. Also, as the server administration functions are included in the Control Panel in this view, the Server Administration Panel becomes unavailable.
Please note, that all subscriptions which were provisioned from Parallels Operations Automation will be created with limits for 10GB disk space, 10 domains, etc. And only option to change service plan for such subscriptions is switch mode to Service Provide View.