I had the same concern when I received a CO Platinum card recently instead of the signature card. I read a few notes from people that switched their card from Platinum to Signature and got NO MILES, since the new card didn't carry over the original promotion. After the first statement, I did get all the bonus miles from the promo I applied with. So, my advice to you is to wait until the first statement.
There is a chance however, that in this case the platinum does not have any bonus in which case it would be better to change it