In the old days it was Fine-15 to VF-20. Today it's a VF-25 to 30. Calling it EF-40 is grade-flation.
Okay, here is a Mint State 64. An AU would only have a brush of wear on the devices and most of its mint luster. The LIBERTY is a little weak on the OP coin. How does LIBERTY look on this Mint State piece which would really be just as sharp on a true AU.
If it’s in a nice old holder, I’d leave as is. I collect the coin, not the holders or the slab, but I do have some nice old slabs that I’ve left them in even though I know it would great higher