I use a Carnival PVP and when I was in a similar situation (for me it was my first B2B and it was the second leg that I would move from platinum to diamond). He told me he would connect the bookings and I should be diamond on the first leg. Fortunately I was but I did meet up with a friends that used a private vacation planner and she wasn't advanced to diamond until her actual cruise which was the second leg of her b2b
Good luck and have fun