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.
Matsuricon/Glass City Con/Youmacon Cosplay: Armin Arlert/young Armin (Attack on Titan); Ultimate Madoka (Puella Magi Madoka Magica); Fem!Masky (Marble Hornets)