BB height when change 29er wheel to 27.5 wheel
The change in rim size will drop the bike 19mm. 29" are ISO 622, 27.5 are ISO 584. (622-584)/2 = 19. If you run larger tires on the new wheels that difference will be less.

We cannot say what effect a fork change will have unless we know what the crown to axle distance of the two forks is. Knowing the travel (120 vs. 140mm) does not tell us anything about fork length.

Be careful if you are considering a different fork length as it will of course change the steering geometry of the bike. 



Update based on new data ...

Yes, those numbers add together, however, 3.3mm difference in fork length is negligible. The fork sag will be much bigger.

A drop of 20mm BB height from 330 to (approx) 310 is only 6%. Balance that against whatever benefit you are trying to get from smaller wheels.