Clarification on upgrading the OS and using OWC SSD? I have a Early 2015 version Macbook Pro which has 126 GB HD / 10.11.1 El Captain OS on it. I wanted to upgrade the SSD. I decided to go with OWC. Transcend Jetdrive 820 for my Macbook model is on bit higher price.

However, OWC doesn't support El Captain. Whether it is possible to upgrade the OS to High Sierra on the current SSD and then replace the SSD with OWC SSD? Whether that will work?
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.