I think that you could "un-stitch" the arm seam a little (no more than necessary), sew a new piece of lace to the entredeux, carefully trim the old lace away, and then resew the arm seam. Most of the attachment could be done by machine, but as you near the arm, you may want to attach by hand. Also, you might want to try a more substantial, sturdier, piece of lace. I hope that this helps. The dress is beautiful.