Nope, not all transactions. Just those for whonever want to participate in a sidechain.

Its like we all use tomato sauce from the same farm (bitcoin) but some want chunky some want spicy. We get to keep the same tomatoes but have a choice in what kind of sauce.

This is a soft fork, which means nodes and users who don't want thos new feature won't be affected.

The great thing is, for this one last change, we get a layer 2, for most future changes.