It's important to remember that this is very much a one-time deal; once Mew has been transferred from that particular Poké Ball Plus, it can never, ever be accessed again.
In case you needed proof of this, Nintendo Soup has reported that one user, who was unhappy with the transferred Mew's stats, decided to delete their save file in the expectation that they'd be able to transfer Mew a second time and perhaps get better stats.
As you might expect, they were less than happy when they realised the Mew inside the Poké Ball Plus had gone for good.
Here's what Nintendo says on the topic:
Mew can only be transferred ONE TIME, into one copy of Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee!.
Once it has been transferred to a game, it will no longer be available on the Poké Ball Plus to transfer again to a different game.
Deleting a save file with Mew in it will also delete Mew. If you have deleted a game with Mew, you will not be able to get Mew a second time on a new game from the same Poké Ball Plus.
The only solution, should you do this, is to buy a second Poké Ball Plus. Gulp.