@vv 
Perfect! Thanks a lot, that did the trick! Just posting here the original result that I wanted, for the sake of completeness:
 

restart;
a := Matrix(3, 3, [[x, y, z], [y, z, x], [z, x, y]]):         # Matrix to compare with
ex := F(Matrix(3, 3, [[x, y, z], [y, z, x], [z, x, y]]));
evalindets(ex, 'Matrix', u -> `if`(EqualEntries(u, a), A, u));
      F(A)      # Outputs as desired; the matrix a has been recognized.

ex := F(Matrix(3, 3, [[a, b, c], [a, b, c], [a, b, c]]));
evalindets(ex, 'Matrix', u -> `if`(EqualEntries(u, a), A, u));
      F(Matrix(3, 3, [[a, b, c], [a, b, c], [a, b, c]]))
# Output as desired; the matrix has not been recognized, thus there is nothing to change.

Thanks again for your help!

Adrian