Cognitive cellular networks can enable opportunistic network access but their effectiveness relies on adaptive handoff algorithms. However, in cognitive radio networks the usufructuary rights of a secondary user are rescinded due to the unanticipated appearance of a primary user causing potential service disruption. In order to provide uninterrupted service to a cognitive cellular user, we propose elastic handoff as a composite framework of conventional cellular and voluntary spectrum handoffs. As with spectrum handoff, elastic handoff grants secondary users spectrum access while insulating them against the arrival of primary users. On the other hand, it is similar to cellular handoff in providing secondary users service assurance. The setup also offers users multiple network access choices, and affords carriers the means to generate additional revenue by capitalizing on excess capacity. We use a blockchain-based spectrum exchange and smart contracts to implement elastic handoff. Our tests show that user-initiated elastic handoff may reduce call drops by up to half compared to observations from a conventional cellular market, and network-initiated elastic handoff can improve a carrier's revenue maximization prospects.