Flights will normally be on the same day and time every week and the cost should be similar for one or two weeks, so it will rather depend if there are spare seats on the day you now want and if you can persuade your travel agent to change your flights without charging you too much extra. You will presumably need to change your hotel booking at the same time. Your best bet is to contact them as soon as possible and discuss it, but first have a look at the 'terms and conditions' in the brochure. I hope you get it sorted.