Information is thanks to _Ninji on twitter (fella who has been datamining)!
To trigger the conversation about art (and in turn the museum upgrade), you must have donated a total of 60 items (or more) to the Museum. I believe once this is triggered, Redd will appear wandering around your town the following day and he will sell you a real piece of art. Donate it to Blathers, and it will cause the Museum to close for an upgrade the following day. I'm not sure if it will be closed, since I haven't gotten that far yet, but I'm sure someone can clarify on this.
To trigger the conversation about art (and in turn the museum upgrade), you must have donated a total of 60 items (or more) to the Museum. I believe once this is triggered, Redd will appear wandering around your town the following day and he will sell you a real piece of art. Donate it to Blathers, and it will cause the Museum to close for an upgrade the following day. I'm not sure if it will be closed, since I haven't gotten that far yet, but I'm sure someone can clarify on this.