A new symbol will be appearing on some Pokémon TCG cards: the Yellow A Alternate symbol. This symbol may appear when a card is reprinted with a new illustration. For example, the Jolteon-EX that appears in the Mega Powers Collection is an alternate illustration of Jolteon-EX from the Generations expansion, so it has the Yellow A Alternate symbol in the bottom right corner of the card.
In the past, cards like these were typically printed as Black Star Promo cards. In some cases, that caused the card to be legal for tournament play in the Standard format for a much longer period of time. For example: Yveltal-EX was released in 2014 as part of the XY expansion, which is no longer legal in the Standard format—however, the Shiny version of the same Yveltal-EX was released two years later as a Black Star Promo card, which continues to be legal in Standard long after the original card would have rotated out.
Yellow A Alternate cards will be legal for tournament play only in formats where the original card is legal. Any card with the Yellow A Alternate symbol will have the same expansion symbol as the original card, and the same collector number with an added “a” to denote that it's an alternate illustration. (If there are multiple alternate illustrations for the same card, those will be numbered accordingly—28b, 28c, etc.)
The new version of Mega Lucario-EX, also from the Mega Powers Collection, is an alternate illustration of Mega Lucario-EX from the XY—Furious Fists expansion. The original card has the collector number 55/111, and this one is number 55a/111. Since XY—Furious Fists is no longer legal in the Standard format, this new card can't be played in Standard format tournaments.
The addition of the Yellow A Alternate symbol will allow for more alternate illustrations of cards to be created without affecting the Pokémon TCG tournament environment. Be on the lookout for these cards in future products!