Using "yet" and "still"

Question

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.

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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.

Answer 1

Your example:

Those changes need to be made but the plan to make those changes does not yet exist.

is grammatically correct. The implication here is that the plan is expected to exist at some point in the future, but currently does not exist -- perhaps it is the next item on the work schedule.

Your second example:

The plan to make those changes has not yet existed.

sounds quite strained. It seems almost like a line from The Hitchhiker's Guide To The Galaxy or any other text dealing with time-travel, and would not find a home in natural English conversation.

If you wished to make use of still, you could indeed say:

The plan to make those changes still does not exist

or, phrased more naturally:

We still don't have a plan as to how to make those changes

In either of these two cases, as @PeterShor notes, the implication is that you expected the plan to be ready by this point, but for some reason it is not.

Answer 2

existed is past tense, whereas exist is present tense. Clearly, we are interested in whether or not the changes exist now, in the present, not if they existed in some point in the past (which does not even logically imply that they still exist now).