Can I swap stories?
There's a story we committed to in the Sprint. After the developer looked into the ticket, it needed feedback from client. A week has passed and still no answer The developer completed other tickets in the sprint and moved on to pick up a ticket in the backlog. Should we leave the tickets where they are or swap them? We won't be able to complete the new ticket from the backlog before the end of the sprint.
In Scrum, you do not commit to scope of work. Instead, the Developers commit to the Sprint Goal at the Sprint Planning event. This gives the Developers flexibility in the work that they do to achieve the goal, which allows for adaptability in the course of the Sprint as the team learns more or unexpected things happen.
If the work represented by the story (or, in Scrum terms, the Product Backlog Item) is required to achieve the Sprint Goal, the lack of feedback from the client should have been addressed before a week passing. The Developer should be involving both the Scrum Master (as someone who can help remove impediments) and the Product Owner (as the person accountable for maximizing the value of the work done by the Scrum Team) to understand what is happening and ensure that the most possible value is being delivered.
If the Sprint Goal is in jeopardy because of the work not able to be completed, the Product Owner definitely needs to be involved. As the voice of the stakeholders, the Product Owner can make decisions on the best course of action to take.
If the Sprint Goal is not in jeopardy, then there's nothing preventing the Developer from taking new work from the Product Backlog. However, it wouldn't be my first choice. If the Developer came to me and asked me what they should do, my first recommendation would be to see if there was a way to help with work directly associated with the Sprint Goal. If the Sprint Goal was met or no help was needed, move on to other work in the Sprint Backlog. I'd even suggest working on refinement activities or work to improve the team's way of working before pulling something new from the Product Backlog, especially if it won't get to Done before the end of the Sprint.