Use Dan Byler's Defer script:
http://bylr.net/files/omnifocus
Defer will change the dates for multiple items simultaneously, and is great for things like adjusting the remainder of a project when a delay strikes.
Also, don't put on more dates than necessary, especially due dates. A start date on each action isn't really necessary in a sequential project unless you are not willing or able to start on an action before the start date.