(cont. of last post)
Unless we go back to point #2 and one knows that the original mezonos can not possibly be “chal” once an “al hamichya” is said (in which case you would definitely be allowed to make a new bracha on the rice regardless of point #1).
Personally that seems to me a bit shvach. It’s hard for me to imagine that there is any concept of a bracha achrona “cutting off a bracha rishona” (although I could be wrong). According to halacha, the reason you can’t eat anymore is because of “hefseik” not because the bracha was cut off.
I think that if this is an issue, it’s not that the bracha achrona cuts off the bracha rishona, but that a bracha that no longer applies to the original food it was made on can no longer apply to anything else.
But I know of no source for this and it doesn’t sound right to me. Certainly if it’s a food that you specifically had in mind (as Yekke said) and that you already started eating (as Sam said), I would think that wouldn’t be an issue. But I have no conclusive sources either way.