REI will refund you the difference. Same thing happened to me last year. I bought a few items and a few weeks later they went on sale. I just went in with my receipt and they gave me a credit voucher.

On another note, I had purchased some PI shoes, but ended up buying some Sidi's before I ever wore them, I ended up moving etc. I found the PIs in their box and took them to REI for a refund. This was several months after I had purchased them and I no longer had the receipt. They were able to look them up on my account and gave me a full refund.

They have great customer service 99% of the time and a great return policy.