There's real reasons to do this. It stops exactly what HashiCorp is doing, and you are likely giving HashiCorp the ability to take your code without giving back.
You can't change the MPL code to GPL. You can change net-new files to GPL, but the existing files must remain MPL AFAIK (unless you're HashiCorp). IANAL
> When someone combines a file or files licensed under the Mozilla Public License, version 2.0 ("MPL") with a project licensed under the GNU General Public License or Lesser General Public Licenses ("(L)GPL"), the MPL's Section 3.3 allows distribution of the combined work (the "Larger Work") subject to the terms of both licenses, as long as certain conditions are met.
Once a file has been distributed under both the (L)GPL and the MPL, recipients of that file can later distribute it solely under the terms of the (L)GPL, in accordance with the terms of that license. If a project wishes to do this, and not to allow others to use their version of this file under the MPL, the project can indicate its decision by deleting the MPL headers described in Exhibit A of the license and replacing them with the standard notice recommended by the (L)GPL. Copyright notices indicating authorship of the file should be retained.
... So you combine with GPL code, and then, say "I don't want to use the MPL."
Is this really so complicated? All updates to be done in the future are licensed under the GPL.
The only firm that benefits from the MPL here is Hashicorp, which is the company that the fork is being done against. Fork it and be done... It isn't like the MPL code can integrate with the BSL code.
There's real reasons to do this. It stops exactly what HashiCorp is doing, and you are likely giving HashiCorp the ability to take your code without giving back.