There is a "switch the be flipped", it will happen starting at a specific block, announced in advance. Until that block, which is yet to be announced, all transactions are validated using the current consensus.
The adoption of the new version is important because, once we are after the block, if there is no 2/3 majority of the nodes, the hardfork doesn't get implemented. That's why I'm following the adoption curve.
Ooh I see! So we want over 2/3 of the witnesses to run HF19, before the switched gets flipped.
I hope it'll happen soon! I'm very curious to see its effects :-)
Thank you for explaining this!