Blizzard posted a pet battle update in the beta forums today and mentioned the following.
“First, we have no plans to make a new pet charm currency this expansion. Polished Pet Charms will remain the standard currency to buy new pets, toys, consumables, and all of the other normally-available vendor purchases.”
I feel this is a huge mistake. Nothing from Shadowlands should be purchasable with currency you can farm now. It trivializes the rewards and the content of the new expansion. Blizzard introduced a new currency with BFA but their fatal error was lowering the value of the old currency by removing rewards you could purchase.
With the release of Shadowlands blizzard should officially establish a two currency system for pet battles. Currency A (Shiny) should be earned in any previous expansion content and used to purchase all pets/toys/stones/food/items from previous expansions.
Currency B (Polished) should only be earned in new expansion activities and be used for pets/toys in the new expansion as well as stones/food.
Once a new expansion releases all of a players Currency B (Polished) should be converted to Currency A (Shiny). The player loses no value from their currency being converted because they can still buy all of the existing items. The expansion feels fresh and new because rewards actually need to be earned by doing relevant content.