It's not just the Plat line. I called the Plat Premier line today for the same reason as the OP, and after waiting a few minutes on hold, the first agent told me that the Summer Bonus promotions cannot be changed. I asked why others were able to do it, and he said that they were not allowed to make changes anymore. It was very clear he was in a hurry to take more calls. (I really wanted to say "But I'm a diamond guest.... ")

I called right back, and the second agent was amazing and switched me right over.

What I don't understand is why Marriott doesn't just offer everyone a one-time opportunity online to pick the promotion that works best for them (no changes after the initial selection). It could work just like Delta's Platinum/Diamond Chioce Benefits (minus the ability to gift your spouse elite status for doing absolutely nothing).

JMHO