I thought about doing the same thing, switching from the MB Platinum to the regular Platinum when my card came up for renewal last fall. Amex offered me 50k MR points though to keep the MB Platinum open so I figured I would just stay on this card for another year.
One thing to keep in mind though is your "spending ability". If you close one account and open a new one, as long as there is some overlap, I assume your spending ability will transfer to the new card but does anyone know for sure?
My current "spending ability" took almost almost a year to earn. I would hate to lose that.