Lead isn't forward compatible with iron actually makes sense, though?
It's not about replacing so much as "lead does not become iron". She's talking about an evolution of one material into another with training and skill advancement, when the two materials are relatively unrelated in that sense.
Also, sorry to hear about the TL's mother, but ain't got no money. That's why I'm here reading free manga instead of buying manga. Gotta keep the lights on, and if everyone who needed/wanted money for their Patreons for me got some my family and I'd probably be dead right now.