We tried that on one of the boxes and it made no difference whatsoever. In the end we had to take all the servers back to NT4 Sp4, re-install Exchange to Sp2 and carry out a 'loss restore'. As you can imagine the users were not too impressed with losing 2 days emails. There has to be a way of upgrading the Service Pack on an NT box without totally trashing the exchange site. I'm just trying to find out if other people have had any experience of doing this. We are going to test a number of different processes to achieve this in our test environment, I'm just trawling to hopefully save some testing time.