Is it possible to replace a painting in a gallery with another one?

Absolutely. Yes, anything can be replaced. A virtual tour is just pixels after all - which can be digitally edited. The art - no pun intended - is in getting the right scale of the replaced picture/art work, giving it an accurate three-dimensional appearance - with accurate shadows. We can so one step further, which is to create virtual galleries of any design - into which we can place any numner of works of art...