Lacazette has had a very good season and does seem to be getting better the last couple of years, but at 26 he should be approaching his peak. If this an addition it's a good move, Lacazette on the wing, if he can play the team will open up space in the middle for Giroud.
However, if it is a replacement, maybe not. If Giroud leaves, is Lacazette coming in that much of an upgrade? Lacazette scores more goals in a weaker league, but Giroud adds more assists. Deschamps thinks Giroud is the better player, he plays him over Lacazette all the time for France. Plus if Lacazette is a replacement for Sanchez, there is no way this is an upgrade.