Has there been a categoric "Party Locking means no changing/finding new companions post Act1"?
Or is everyone jumping on this because Larian have previous in DOS2 and stated they want you to choose out of the initial companions which ones to go forward with?
In Community Update 7, Larian said
Will companions be interchangeable during long rest?
Yes, at the start of your adventure your recruited companions will be at camp when not in the adventuring party, and can be swapped in and out at camp. Just like friends in real life! After the first act however you are going to have to commit, also just like in real life.
"After the first act you are going to have to commit" combined with how DOS2 worked implies that the companions you don't choose will be killed or otherwise turned into antagonists.