Wedding etiquette question: can I bring someone else in place of the guest I listed on my RSVP card?

My friend’s wedding is coming up and I RSVPed to go with my boyfriend about two months ago. Unfortunately, my boyfriend absolutely cannot get off work to attend, so I thought I’d invite another friend to go with me. The friend who’s getting married has met my other friend before, but only briefly. Is it okay if I bring her instead? I’ll go alone if I have to, but I won’t know anyone else attending apart from the bride and the maid of honor.