thank you. AFAIK they cannot re-license the past so it would be possible to continue using the Hashicorp versions prior to adoption of the BSL while migrating to an alternative or even fork if there is a community willing to continue development in an OSS fashion.
And I completely agree with you. Hashicorp's change of license is, in the words of @bcantrill, "shitting in the pool of Open Source pool and disgusting corporate behaviour".*