Assuming that you haven't paid for the bridesmaid's dress and the bride has paid for it, she may be quite hurt that you don't want to wear it for very long on her big day and would rather ditch the dress at the first opportunity. I guess that it all comes down to personal choice but I wore my dress for the full day and evening - as did my bridesmaids - as I figured that as my hubby and and I had spent so much money paying for this one day, we were going to milk it for all it was worth ! Seriously though, you only get one shot at wearing 'the big white dress' (unless you are a serial bride) so it seems silly to change out of it as soon as the reception starts. It's what makes her stand out as the bride on her special day and similarly, what identifies you as part of her bridal party by wearing that dress. It's only for one night and as has already been said to you, you will get plenty of opportunities to wear your other dress to something else. If you want to test the water though to guage her reaction, you could drop hints by saying something like ' How do you think you will manage in that big dress all night? Will you be changing into something more comfortable later?' If she is adamant that she will not be changing as she wants everything to be perfect for her big day, then personally, I wouldn't change, I would respect her wishes as it is her big day after all.