I’ll let my colleague who created the script help with the first part of your question, but for the last two questions:
No.
The script is one option; manual migration is another option. That is why we provided very clear descriptions of how the theme component’s settings / strings map to the welcome banner’s settings / strings.
December 15 is when we are completing our migration of hosted customers and when we will officially stop supporting / maintaining the Search Banner component. We are recommending that other users of this component migrate before then so they’re not caught off guard when the theme component becomes incompatible with Discourse core in the future.
If folks choose to wait until later, they should still be able to run these tasks or perform the migration manually, but in the meantime they will be running an unsupported theme component.