I see the advice has been given and received, so there is no further need for comment on the dresses, but I gotta say, I admire your boldness and fun! And you must be good in the figure department too, as, on me, that second dress would say "HEY, every one LOOK, look at all the weight I've gained over the years all round the middle! HEY, did ya hear me? Look at it!" So good for you girl!!!!
I think it's up to you about the changing, but I've been to plenty of church weddings and don't think you need to worry about wearing a floor length gown. If it's strapless I'd wear a matching shawl or sparkly sweater though. I think you can find something that feels like your classiest self that will allow the bride to take center stage in family pictures, but is still fun and all YOU, and something comfortable for all day would be easier. It is a very long day!
I've been a step-mother of the groom, and then bride, and both were far trickier trust me. I was early 40s. Did not want matronly beaded jackets but didn't want or have a figure for teenager at the prom, and then not wanting to outshine the kids' mom, but still trying to look good...... It was a tightrope finding those dresses!
Good luck and have fun!