Did you only open up the seam on one side? How did you keep that side from unraveling...is the sewing up the triangle sufficient enough to lock the preexisting stitches? I hope my questions make sense. I just learn to thread my machine yesterday so I am REALLY new.
I opened the seams on BOTH sides because that way you can style the dress plenty of ways. And sewing up the triangle and trying to fold that area down was a little complex, but I was able to get it to work. You could also do a zig zag stitch off the edge OR you even make yourself/buy some binding tape. And to "lock" the stitches you took out at the end of your triangle, you can just sew down a little and then back stitch to prevent your dress isn't going to come apart/unraveling any further. And when you sew that part, try to sew on the original seam to prevent from making a new seam.
Hopefully, my text make sense. It's so hard to envision sometimes. Sorry in advance. :/