That is interesting to know that the 15 nights didn't go away. When I canceled my regular Marriott Visa card a few years ago, they took away my 10 night bonus very quickly.
But from my experience that you will not get an additional 10 nights added to the 15 if you get a standard Marriott or business Marriott card. I got the Premier with the 15 nights, and then the business. The total nights I got for having both cards at the same time was 15.