Can I Change the Dates of my Trip (same trip just different dates)
Yes you can change the date of your trip; that is usually possible if there is availability on the trip you want to change to.
You can change the date of his trip but there will be a $150 change fee* for the airline ticket, plus any difference in airfare, if applicable, and you will also have to pay the standard $50 change fee to a new date. So, generally speaking, minimum change fees will be $200.
*in most cases the minimum change fee for airline tickets is $150, although there are exceptions.