To play 1NT - 2
as diamonds is perfectly legal at Level 2 or higher. All responses to 1NT are permitted. There are, however, two snags.
First, you may not call the 2
bid a transfer. If 2
is a transfer then it shows hearts unambiguously. If you play it as either hearts, or weak with diamonds, that is how you must describe it, both on your convention card and in answer to questions.
Second. some people confuse this with a defensive sequence, where 2
is an overcall. Suppose you agree to play Asptro, where 2
shows spades and another: you may not use the sequence (1NT) 2
to just show diamonds at Level 2 or Level 3.
Furthermore, if you want to play (1NT) 2
as either spades and another suit, or just diamonds, at Level 4 where it is legal (since any defence to 1NT is permitted at Level 4) then you may not call it Asptro: you must describe it on your convention card and in answer to questions as either spades and another suit, or just diamonds.