Don't make the same mistake I did.
I saw the number of points I will be receiving for the Premier Plus upgrade and without all the full details released as of that time, I applied for the Premier Plus. I came to realize (after asking the question on here), that I will not get my 15 night credit this year as I upgraded prior to my anniversary date.
I called Chase Marriott to cancel my application right away but they couldn't see it in the system. The woman I talked to said once it was processed, call and they can reverse me to the Premier card as I was an existing holder.
I got my Premier Plus card today, called Chase Marriott, and they said that is NOT the case and I can't go back to the premier card as they don't offer it anymore.
Looks like I lost my 15 nights and need to figure out how to make it up...