Starting from 5th June 2020, a serial code that allows users to redeem a Dynamax Crystal for Gigantamax Copperajah has been made available.
As far as I can tell, the event were announced in European countries, between going to Gamestop or GameMania between 5th June to some early date in July(prior to the DLC launch). (link)(link 2)
The changes done to the Dynamax Crystal populated dens was first observed here (June 02, 2020),
however it's status of it being an official event w