I think a lot of it will end up being sold. Unless retailers are prohibited from selling it after April 1 (which may be the case, I honestly don't know) you will probably be able to pick up old Reebok jerseys off the clearance rack at the store.
What will all the team shops do with all the leftover Reebok apparel come 1-April ?? I haven't seen any fire-sales on Reebok apparel at the shop at NFL.com or BILLS.com ...
Most likely they will be forced to destroy it. However, Reebok hasn't produced any new goods this year because of the contract change, therefore most of the stuff has been in a fire sell all year long.
I'd assume a lot of left over Reebok stock would get donated to third world countries, just like locker room t-shirts of losing teams.