The question is hard enough to understand, but I think you mean you need to migrate Exchange from one box to the other?
If this is the case, the choice is dependent on if you can take any downtime and how big your mail store is. When we did it, I didn't have the option of downtime. I had to setup a mail frontend to handle request for the clients and then start migrating the individual mail stores across. I could pick times when individual users were likely to be sleeping to do the changes meaning on two people noticed the moves.
If you can take downtime, it is possible to backup Exchange and restore onto the new box. If it doesn't work, you can just power on the old box again.
For the second question, Active Directory is better to spin up on the new box and let it propagate. Once it has, you promote the new box into the primary rolls and remove the old one.