When someone says,
The changes have to be updated.
someone may reply,
Those changes need to be made but the plan to make those changes does not yet exist. (as sometimes found)
Is it grammatically correct? Why shouldn't it be something like the following?
The plan to make those changes has not yet existed.
Moreover, sometimes "still" is seen in place of "yet" such as.
Bill has still not arrived.
Shouldn't it be
Bill has not yet arrived.
I'm not talking about the conjunction (yet/still) used to connect two simple sentences.