I don't know if their lacefront wigs have a mesh cap like their ponytail/pigtail wigs do, but if they don't, that alteration method will not work. It's similar, but not quite the same. I've messed up a wig trying to do that.
You'll want to just cut the elastic at the sides. Use a seam ripper to remove a couple of the wefts temporarily, cut off however much elastic you need to, sew up the space between, then sew the wefts back on.
Conventions for 2015 (so far...): Momocon, A-Kon, Otakuthon, Sac Anime
Cosplay line-up: Mayoi Hachikuji (Bakemonogatari), Nona (Death Parade), Nagisa Momoe (Madoka Magica), Menma (AnoHana)