Qube 6.x licenses are not backwards-compatible with previous versions. Nor will licenses from a previous version of Qube work with Qube v6.0
We've gone to a node-locked licensing scheme with perpetual (non-expiring) keys instead of the annually expiring, non-node-locked keys that were used before. Also, each license will be tied to a specific major version of Qube; eg, 6.x, 7.x.
All Qube customers are entitled to key refreshes forever, regardless of whether they're still under support or not. If your customer needs their 5.x keys updated sooner than before the end of the year, please contact your sales rep and they will start the ball rolling.