Yes no problem.

In fact even if the coupon says, for example JFK you can reroute to EWR or LGA for no charge since these are what is known as "co-terminals".

However, if you think you might want to do this then it does minimise potential questions from check-in staff if you get the coupons issued with the city code rather than a specific airport. Of course in some cases the city code is the same as one of the airports - e.g. SFO or LAX - and in that case you have to just hope it isn't a problem. In the past I have had no difficulty, for example, using a coupon that was originally for a specific SFO-ORD flight on a SJC-ORD, and similarly LHR-JFK on a LHR-EWR, and LHR-IAD on a LHR-BWI.