In reverse order, I don't think that going from a 6" pot to either a 7" or 8" one is a problem either way.
The aerial roots are structurally different from those that grow down in the medium, so burying them is not a good idea. On the other hand, I have found that phalaenopsis, once repotted to a larger container, will often have those aerial roots start to grow down into the medium, and as the parts that grow there are tailored to that environment, they're fine.
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