Vic. 'If you are asking what the p-value of the 600 trial will...

Vic.
'If you are asking what the p-value of the 600 trial will be if the 1000 p-value is 0.01, well it depends on how the additional 400 compared to the original 600. If they are the same, the p-value of the 600 will be more than 0.01 because the sample size has decreased as the standard deviation of the sampling distribution has increased and made the distribution 'fatter'.'

Yes this was what I was asking, and sorry for not articulating in one sentence as you have done. Based on the assumption that the final 400 will yield the same data at the original 600, is it possible from stats to predict the P value for the 600? I understand it would be higher than 0.01 as it would be less powered, but I have no idea as to how to calculate the number.