Once Pokèmon Bank has been updated, you'll be able to use it to transfer Pokèmon you've caught in the Nintendo 3DS Virtual Console versions of Pokèmon Red, Pokèmon Blue, and Pokèmon Yellow into your copy of Pokèmon Sun or Pokèmon Moon. Pokèmon from Pokèmon Omega Ruby, Pokèmon Alpha Sapphire, Pokèmon X, and Pokèmon Y can also be brought into Pokèmon Sun and Pokèmon Moon in the same way.
You will be able to use your existing Pokèmon Bank account to transfer Pokèmon from Pokèmon Red, Pokèmon Blue, and Pokèmon Yellow to Pokèmon Sun and Pokèmon Moon. Due to certain limitations, it is recommended that you install Pokèmon Red, Pokèmon Blue, and Pokèmon Yellow on the same Nintendo 3DS system as Pokèmon Bank.