Yes Costco will swap the M&RIE receivers in and out for you with no additional costs. That was my initial intention as well if I did not like them. Turns out for me they were the best choice. I would think they would be happy to let you try them around the store. I took mine home for a week trial but I decided to keep them quickly.
I have M&RIE Directionality in the Music/TV and Outdoors programs. I did not get it in the All Around or Hear in Noise programs. In the All Around program the default 360 Directionality mode uses the M&RIE Receivers automatically in low noise situations. I believe you need to talk/discuss with your HCP about what programs you want M&RIE Directionality activated in.
I added M&RIE Directionality to the Music/TV and Outdoors programs when I came back the next week because I forgot to ask initially. It worked out. I was in enough situations to know I wanted them in more programs. Maybe you would want to do this initially if you are going to try them out while you are at Costco?
Some people use M&RIE Directionality in the All Around program as well but I have kept the default thus far. I think the new features in the EP 20S, as opposed to the EP10s, All Around program are more active this way.
I hope you enjoy your new hearing aids!