Once the cards stop being printed, you'll have "stake" DEC to them in order to unlock them. From there, you'll be able to burn, trade, or sell them.

First you earn them, then you pay for them, then they're yours to do what you want.

@splinterlands/announcing-the-new-chaos-legion-reward-cards

"We also intend for the cards to be able to become unbound - meaning they will be able to be transferred, rented, burned, or sold - once they stop being available in loot chests, by infusing them with DEC. We estimate that roughly 1 - 2 years out from this initial release of soulbound reward cards that are associated with the Chaos Legion set we will release a new set of soulbound reward cards that will be associated with the Rebellion set.

At that point, the current set of soulbound reward cards will no longer be available in loot chests, having been replaced by the new set, and the old set will then be able to become unbound by staking DEC into the cards, a portion of which can then be reclaimed later by burning them."