changing specialty

If I change the specialty for the second time, will nss go back to the numbers I see in horse's base stats ?

yes, converting the specialty adjusts everything, including going back to base stats in any of the nss stat categories of the new discipline. 

Thanks, that's good to know ! I get slightly annoyed about the fact that if horses between 2 and 3 get stats from rides, those stats are allocated totally randomly and when discipline is set at 3-years-old, those stats don't get shifted so as a result nss are higher than base stats... It seems that the only way to shift them away from nss is to do second conversion...