Swapping 6+ to 6 Question

There are iPhone 6's available to pick up at my local store. I'm going to go there tomorrow and exchange the 6+ for a 6.

Will I need a Genius Bar appointment for that or will they be ok with a walk-in? I tried to make an appointment but there wasn't an option to choose one for tomorrow.

And if they have the 6's in stock, will it be possible for me to swap them out there no problem? Never done this before and I really don't feel like sifting through all the threads already made about this topic

Oh yes and I bought the 6+ on apple.com via pre-order on a 2 year contract through AT&T if that makes any difference.

 

It doesn't matter why.