"Bigbird 123 can you accept 13L"
"Affirm, Bigbird 123"
"Bigbird 123 runway 13L cleared to land"
Which case is that? If it's a visual approach, what precautions have been taken to ensure "that the meteorological conditions are such that with reasonable assurance a visual approach and landing can be completed", which is a requirement of the visual approach. The pilot has not explicitly accepted a visual approach.
I contend that it is abandoning the published instrument approach for a visual approach. Since we don't know that circling minimums are published for the approach in question, or that circling is allowed for the approach, I believe you are switching from one approach to another.
Also, it is poor ATC terminology, and serves to cause the type of confusion being discussed here.
You accepted a visual approach when you said "Affirm." Since you were not cleared for an instrument approach to the new runway, the ATC controller must assume you have the airport and new runway in sight. Also, the OP clarified that he was indeed asking about a visual change of runway.
If you want to contend that it is a circling approach from the original instrument approach procedure (assuming it has published circling minimums), then you must accept that the published missed approach procedure applies unless specifically instructed otherwise by ATC.