OTOH, uniforms don't change overnight elsewhere in Trek. Some people used the second TNG style as late as "Tears of the Prophets", and the Admirals always seemed to be out of synch with the line officers. So we could argue that the dress uniforms of ST:INS hadn't yet permeated to the whole fleet - or that they were to be used in "civilian" parties only, including diplomatic occasions with foreign dignitaries, or private weddings.
Actually, that bit about Admrial's uniforms also reminds me, Insurrection featrues Admiral Dougherty wearing the new gray shoulder Admiral's uniform, which hadn't been introduced before Call to Arms. In fact, A Time to Stand features Admiral Ross wearing the old style Admiral's uniform, inctoduced in TNG S6. Of course, TNG S6 still featured a few Admirals still wearing the previous design.
Please excuse this post as nothing more than a shameless attempt to raise my post count.