Shoes wrong size - will shop take them back?

Hi,
I recently bought a pair of Asics 1000 in one of the Irish sports chains as a replacement for an old pair of 1150s. I was happy with the sizing of the old shoes, but I tried on the 1000s in the shop just to be sure. Thought they felt a little bit tight in the same size, but I figured that with my usual (thinner) running socks & adjusting the lacing properly, they'd be ok.

Two short runs later & I've had pain in the ball of my left foot, which I believe is often due to badly-fitting shoes. Tried the old pair in between those two runs & had no problems. Given the shoes have been used & that I tried them on in the shop, I presume the shop is under no obligation to exchange them for a different size? I'm going to chance it & see what they say, but was wondering what experience forum regulars have in exchanging shoes.