Magic: the Gathering often reprints cards in specialty products like Commander Masters, and products like these are the perfect opportunity to modify and update those cards. This sometimes means getting new card art or special foil treatments, but the most substantial change is rarity shifting. Reprinted cards can be upshifted or downshifted in Masters and other draft-oriented products, and there are several good reasons for this.
Source：https://www.cbr.com/why-magic-masters-sets-shift-rarity-for-best-cards/
