Greetings Bhanu,
The Priority will only affect the start and end dates if you had defined start and end shifts in the configuration for Priorities.
For Notifications, the new dates will be recalculated every time you change the priority. For Orders, the shifts are only applied once, and only if you specified the Priority in the initial screen in IW31. Maheswaran already explained that.
If you create the Order from Notification, then the relevant Notification dates are transferred, as Pete Atkin points out.