Why must the object of knowledge be "changeless" and "eternal"?

Plato's route to his theory of forms takes him to the surprising conclusion that the object of knowledge cannot change.  Surprising though it may be, it is a direct consequence of the conclusion that knowledge requires that what is known be necessarily true.

Plato's argument goes essentially like this:
 

Premiss 1: If the object of knowledge were to change, what is true of it at one moment in time might be false at another moment in time.

Premiss 2: If a proposition, P, is necessarily true, then it is not possible for P to be false at any time.

Premiss 3: If "S knows P," then P is necessarily true.

Conclusion: If S knows P, then P cannot change truth value.

But by the assumption of the correspondence theory of truth:

.If P is true, then there is a certain state of affairs in reality to which P corresponds which makes P true.

Therefore,

          If P is necessarily true, then the state of affairs to which it corresponds cannot change.
 

  Thus since knowledge is necessarily true,  the object of knowledge cannot change.

Insofar as everythingf in the physical world can change, Plato's serach for knowledge now leads him to postulate another "world" of eternal changeless objects of genuine knowledge, and he calls the objects in this "world" "eidoi" which is best translated as "Forms."  Thus he is led to his Theory of Forms.