Identity and Interchangeability

According to Kant, from the proposition 'A = B', it does not follow that 'A' and 'B' are interchangeable in that, or in any other, proposition.  So, perhaps Frege's point is that from the sentence 'The Morning Star is the Evening Star', it does not follow that 'the Morning Star' and 'the Evening Star' are interchangeable in the utterance of that, or any other, sentence.  Accordingly, what that example primarily illustrates is that 'meaning' can be equivocal, i. e. can be sometimes synonymous with 'Reference', sometimes with 'Sense', regardless of whether or not the problem further serves as the foundation of a theory of Semantics.  However, a Fregean might reject this diagnosis of the example, because it relies on the term 'interchangeable', which has a practical connotation, which is not easily accommodated in the Fregean Philosophy of Language.