I bought some coins immediately after my account was verified. No problems (except the slow verification).
But now they ask to apply for a corporate account like I never verified my account as corporate account...
Maybe something in the process of upgrading their systems caused the problem, I don't know...
4 years is a long time, documents you provide probably expired and need to submit up to date documentation.
I don't think there's a rule if you verify an account once, it stays verified forever, things change and need to be updated.