You didn't mention how you booked your trip, whether via Couples directly or through a travel agent. I would start with contacting whatever group you booked with and ask the question.
Be prepared for rate changes as CSS tends to be less expensive than CSA and depending on when you booked, you may have had a special rate that is either no longer available or may no longer apply given your travel change. One thing to think about is that if you booked with a promotion giving you resort credits for a 7 night stay, you will likely lose them if you change to less than 7 at either resort. you also need a minimum of 3 nights at each, since a "split stay" is actually considered 2 separate stays. There are several threads on the MB relating to how long or short a stay is worth splitting for, especially between the Ocho Rios and Negril sides of the island which you may want to check out.
We are traveling in that same time period and spending 4 nights at CN and then moving over to CSS for 4 nights, so maybe we'll see you at CSS! Good luck!
We always go for two weeks and always split our stay. But we split on the same side of the island in order to avoid the lengthy transfer from one side of the island to the other. We will split between CSA and CN, or between CSS and CTI (or sometimes a resort in the Montego Bay area the second week as it is close to the airport). When we have gone for only one week, we always do the exchange day to visit the sister resort on that side of the island. Certainly you have to also consider what kiki23 said earlier in this thread about changes to your reservation. Trust all will work out as you wish -- enjoy...