I have 5 privet bushes, in a U formation, which originally formed a
hedge, but they've been mistrimmed, especially the one in the middle.
It won't grow big enough to reach the ones next to it.

I'd like to dig that one up and plant a new one, which I figure will
eventually grow big enough.

How much do you think I have to dig up? I figure part of the root is
woody and will take a long time to rot, so how much of the root do I
have to dig out and replace with soil or topsoil or whatever?

Thanks