No, they do not. They can release new versions with some other license if they get all contributors to agree. They absolutely cannot pull the FLOSS licenses from existing releases.
Contributors must sign a Contributor License Agreement (CLA) before a contribution will be merged. The CLA signers gave Elastic the right to change licenses, among others, when they signed the agreement. Consequently, Elastic doesn't need to get individual sign-off from each contributor before changing the project license. The code wasn't based on copyleft license, so there's no compulsion to continue using the same licensing terms for all future distributions. That's a motivating factor for many CLAs. If you made a contribution prior to the introduction of the CLA then things get murkier.