I'm pretty sure that when I hit Tom Nook's shop with the Golden Shovel that that is what made him say that they are expanding!!
So I read that when you hit the shop 3 times on the back, and 2 times in teh front, that it will make him wear PJ's, but when I did it, I hit the front first, twice, and the back second, three times.
That day I only bought one thing, but after that I left the store, went back in a few minutes later and sold some stuff.
What I'm saying is that I'm thinking that this is either a glitch or I'm confused... Could someone try this and tell me if they got the same thing? Of course your shop can't be already upgraded. Please try!