How to support two different Yocto branches (kirkstone and scarthgap) concurrently?

I started using Yocto some times ago for custom Linux projects ; I own two meta-* which I based on the kirkstone branch.

Now, I would like to maintain the kirkstone branch until it reaches its end-of-life and provide a variant for the scarthgap branch.

• should I do a massive import of my source files from kirkstone to a new branch scarthgap, commit and then adjust?
• should I cherry-pick existing commits and then apply on top of them a few adjustments?
• should I cherry-pick commits one by one and adjust as the process goes?
• should I fork a scarthgap branch from the top of my current kirkstone branch and then make some adjustments?
• ...

What are the best practices for such situations?

EDIT: This post is about the code base maintenance to support multiple distinct branches of Yocto.

Let's consider the following case: I started a project on the kirkstone branch of Yocto. My recipes are thus based on this version. Now I would like to "also" support the scarthgap branch. This later version of Yocto introduces some changes compared to kirkstone.

My question is how can I efficiently make my code initially made for kirkstone compatible with the scarthgap changes. I would like my project to have a dedicated scarthgap branch to hold this specific version (specific in the sens that it benefits from the latest Yocto changes).

Normally you'd branch your kirkstone branch onto a new scarthgap branch and make the changes needed there to support the new release. You'd then add changes as needed to either branch. Kirkstone shouldn't need major changes since it is an older LTS that doesn't change too much.