(I'm not sure if the priority exchange also applies to the "Create Your Own" subscription option. Maybe someone else here knows.)
I haven't yet received a brochure for the 2013 "Take 3"/"create-your-own-make-3-or-more" series, but in the brochure for 2012 it does say that ticket exchange privileges are available.
The main problem with the exchange option is that you have to be able to get to New York to go to the box office. They will do a mail-in exchange, but you don't have any say about particular seats that way either. (And, of course, you have no input on seat selection when you order a subscription either.) I called them last year and asked if I could do a telephone exchange in the week before single sales, so I could say which seat(s) I preferred, and was told "no." I don't know if the exchange privilege includes asking for different seats in the same section at the same performance and it's not clear from the brochures if that's included.
As far as I can tell, the per-ticket cost of a subscription is no cheaper than just waiting for single sales, at least before "dynamic pricing" kicks in down the road.
On the bright side, I pounce on-line on the first Sunday in April when single seats go on sale and have never had any trouble finding lots of great seats to choose from, even for the most in-demand performances.