What are the changes between BLtouch 3.0 and 3.1?
Best reference would be the original equipment manufacturer (Antclabs). It appears their sensor has had many versions:

So, from 3.0 to 3.1 is a software upgrade. Reported highlights are:

★ Smart V3.0 (Mar. 2019 ~ ) : S10, S60, S90, S120, S140, S150, S160,
PWM Wiring defect indication(Blue LED), QR, Molex 1.25, Plastic
Push-pin(PC), Power Wiring defect indication(Red LED)
★ Smart V3.1 (Mar. 2019 ~ ) : S10, S60, S90, S120, S130, S140, S150,
S160, PWM Wiring defect indication(Blue LED), QR, Molex 1.25, Plastic
Push-pin(PC), Power Wiring defect indication(Red LED)

If you look at the manuals you will see the differences:
V3.0

V3.1

As can be seen, timings are different and more PWM position G-code options are available in the V3.1.