Why does Kaiki keep changing shirts?
As you can see, the interior coloring of the cafe changes too. We could assume that they're moving together to a new place every now and then, or that the cafe decorator is currently working 400% with customers still in the cafe. Senjougahara also changes her "disguises" between scenes. We can assume they have enough money to afford all that, and are changing between their meetings.

But what's really going on is probably just Shaft doing a comedy thing with changing decorations to somewhat lighten up the mood of a very serious discussion between Kaiki and Hitagi.