Clarification on upgrading the OS and using OWC SSD?
Where does it say OWC doesn't support El Captain? I can not image why the version of macOS would matter. I assume you would put the new drive in the computer and then install the current macOS. Afterwards you would run the Migration Assistant to copy your files to the new SSD.

In other words, I do not see the need to change or update your existing drive before swapping with the new SSD.