Of course you can fly the first sector and then re-route. It should cost no more than usd 125 plus maybe differences in taxes/fuel fines.
The problem occurs when you change your first segment: regardless whether you change the rest of itinerary or not at all, if the prices have gone up since ticket issue, you are going to pay the difference.

I believe the rules have changed here this year: i have changed the first segment of an AONE4 and some other segments of the ticket, but ticketed points/stopovers etc remained the same, it was always just a date change. Basically, i had to postpone my ticket. There were no charges whatsoever then, but when i read the rules correctly now, i would have to pay if the fare would be higher now.