I just happen to have found it. In configuration.h use ctrl F to find this line: #define DEFAULT_AXIS_STEPS_PER_UNIT {64.25,64.25,2560,600}
The pitch of 8mm thread is 1.25, the new pitch is 8.00. So the printer now needs to make 8.00/1.25 = 6.4 times less steppes than before.
And 2560 [steps/mm] / 6.4 = 400 [steps/mm]. Change 2560 in 400 and you are set!
I am suprised in how I could not find this on the internet.