Why is it necessary? It's ugly, but it's earning millions. Find out why resistance to change is huge. Maybe the devs are stuck in their ways, or maybe it's too damn scary for them to fail because the business comes down hard on them? Get to know your devs, find out from them what they think the problems are and how they would like to solve them. Maybe they just need more help. And as the team lead it sounds like you need to bring the "business unit" up to speed on the current reality?