Being the Product Owner responsible for expanding or modifying a mature and stable system is quite different from being responsible for developing a new system for a new process, service or product.
When you are expanding or modifying a mature and stable system, your main responsibility is managing the Product Backlog and the Product Backlog items. If your approach is Value-Driven and you keep a keen eye on stakeholder management, your success as a Product Owner is guaranteed.
When developing a new product, you will face increased demands. As the owner of this the new product you are the one who has a vision and you know (or think you know) how business problems can be solved and business goals achieved with your product.
It is not just about defining and clarifying requirements and managing the Product Backlog any longer, but it is about managing the Product as a whole, the big picture. It is about Product Management. What direction is the market moving into? What trends must be observed? How do we respond to that?
To know that you are still on the right track you need frequent feedback from the business itself, the users and the market. Thus you will be able to quickly detect changes that impact your goals, and be able to respond to them. Frequently delivering an increment of the system will automatically provide you with the feedback you need. This is also the way to create business value while still in the process of developing the product. That is why release management is also a responsibility the Product Owner faces.