I've heard some people discuss that the contribution agreement that Hashicorp makes people sign gives them the right to change the license for existing contributions, but I'm not a lawyer so I really couldn't say for certain either way.
However the MPL and licensing in general is irrevocable. They have irrevocably licensed Terraform 1.5.5 under the MPL and an enterprise license (dual license). Anyone can use, modify and distribute version 1.5.5 under the terms of the MPL.
Since HashiCorp retains full copyright they can release the next version under the BSL.
Note that many free software projects (like Linux) don't have a CLA which makes relicensing impractical since every contributor would have to agree to it.