They are only required to put an expiration date on it because of the SPF. BE doesn't actually expire so you should be fine. With that being said, I would be a little angry if something I paid full price on was set to expire soon.
Maybe you could go back to explain the situation. Explain that you just want an even exchange, not a refund. Even though you opened it, it's still going to be full and obviously unused.
Sephora has a very liberal refund/exchange program. I'd give taking it back a shot. If you're in their Insider's Club your purchase would be in their system.
Actually, I bought a lotion from cosmedicine that was expired and sephora won't take it back. They said that makeup lasts 3 years after the expiration date, and if it lasts less than 3 years, you'd see the number of months listed on the product.