Hi, @homeenglishuk . It’s been six months or so since I did this, so things may have changed, but…
At the time I did this, it wasn’t possible to transfer a traditional Wix Premium plan over to Editor X using system tools. It was necessary for me to (1) purchase a new Premium plan for Editor X and (2) then get a credit (with the help of support) for the value remaining on the Wix site.