I think many would be very happy if there was a time limited option to redo inventory upgrades ONLY during a major rebalancing, myself included. This would keep people from feeling the frustrations when a major rebalancing is announced. It would also be a fair compensation without having much refund attempts like, I'm sure, many will try to do. 

The rebalancing option can be such as returning a portion of credits and acoins. It doesn't have to be returning equipment and mechs, just the credits, acoins, and maybe parts used in upgrading, not purchasing new items.