I’m on a 26 Cotic Soul and my old 120mm forks have just died and are now being replaced by some Revelations with 140mm. I like the way it rides currently but I also like the idea of the bike being a bit more agro with the 140 Revs on it.

So… how much do I have to lower the bars by to have a similar feel to the 120mm set up (or even better for climbing etc)? I have about 30mm riser bars and 15mm of headset spacers paired with a 70mm 0 rise stem.

I am fully prepared to swap out both bars and stem.

Depends.How much sag you run.How much compression damping you run.
I went from rigid to a 100mm fork and it took a while to get the bar height right.It changed again when I started using bottomless tokens (as I could run a bit more sag).
Go ride it.Arse about with the bar height till it feels good.