not the way they were used back then by Momo's grandpa
You read it but didnt understand it judging by what you are still trying to push,
let me give you a quick grama course,
meito are "ranked" from supreme, great grade, and so forth and within those "ranks" they are ranked by the strongest to the weakest.
Mihawk has already told us that Yoru is ranked 1st in the supreme grade, what Momo's grandpa said was that Enma under Zoro can rise its rank to be the strongest great grade blade, never did he ever EVER ever say that Enma will magically change from a great grade to a supreme grade
Cap. We know what you say is cap since it happened to ryuma. Hence why even as a black blade shushi is the same rank as enma. Same with wado. It was stated that wado, enma and shushi (upon turning black) were all the same grade and that as a result enma can become stronger since it wasn't a black blade.
Now let me educate you on the English language.
Grade: a particular level of rank, quality, proficiency, or value
Rank: a position in the hierarchy.
So by defintion grade means: "a particular level of position in the hierarchy, quality, proficiency, or value"
using logic if enma is already the strongest of the greest great blades then its grade rises to a Supreme great blade when its rank improves.
the same magical way in which shush became a great great blade when it turned black is the same magical way in which enma the strongest sword of great great blade will rise to supreme great blade when turned black