When do you remove/replace mulch?

I’m at the mulching stage of a flower bed renovation. The old mulch looks tired. But there’s still 3-4 inches of old mulch there. It is more broken down as you go down, but there’s still quite a bit there nonetheless.

does that mulch need to be removed? Thankfully it’s only a relatively small area, but the thought of it still stinks to think about.