From this last source, we see the following:
1. There is a difference between a situation in which he had started eating the second food and a situation in which he had not.
2. If he hadn’t started eating the second food yet, then it is possible that he has “separated himself” from the bracha rishona once he made a bracha achrona. It is a safek.
3. If he had started the second food, the original bracha rishona is still chal as long as the bracha achrona is not chal on the second food.
4. Borei nefashos said on rice is not chal on a mezonos (even though bdieved borei nefashos might be chal on anything)
5. Al hamichya said on a mezonos product might be chal on rice even if that wasn’t his intention (as seen if 37/3 quoted earlier)
6. Borei nefashos said on rice might be chal on less than a k’zayis of mezonos.