Well..Thanks for your reply.
I checked the following things and found no problem at all

My origin I/B is SEA-MSP-JFK-CDG-BUD but want to change SEA-MSP-IAH-CDG-BUD.
I asked one of my friend who works at a travel agency and price the itinerary what I want with a past date ticketing date and found a same fare basis what I ticketed appeared

But delta doesn't want to change my ticket at all !!