Board Thread:Questions and Answers/@comment-28340828-20180616205329/@comment-24900075-20180617003444

If a High 1-A disappeared, that still shows a massive weakness of being able to be defeated. An omnipotent or a nigh one CANNOT be defeated like that. It's not an assumption, the simple fact that a High 1-A disappeared (and likely for good since Character A lost its weakness. Oh and disappearing is also the same as dying in this context so I have no idea what the hell you're talking about) means that Character B was never High 1-A. And if Character B was never High 1-A, Character A would've never been High 1-A since Character A was at least somewhat stalemated by Character B, a strong 1-A.

In short; if the High 1-A that disappeared (As in was defeated) then they weren't High 1-A and therefore the other character is also not High 1-A nor omnipotent.

Now a more direct answer to your question; no. If you were High 1-A and your weakness disappeared, that wouldn't magically make you Tier 0 since the very fact that you once had a weakness makes it so you aren't tier 0. A tier 0 is and always was a Tier 0, there's no jumping or upgrading tiers to it.