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.
Fandom Fest 2014 cosplay: Kiki (Kiki's Delivery Service), Sayaka Miki [mermaid] (Madoka Magica)
MCM London Comic Expo cosplay: Kiki (Kiki's Delivery Service), Nagisa Momoe (Madoka Magica)
ALA cosplay: Kiki (Kiki's Delivery Service), Snow White (Zombie Panic in Wonderland)
2015 conventions: Anime Los Angeles, Anime Central, Japan Expo