If you established a Roth IRA when your AGI was below the threshold, what happens if your AGI now exceeds that amount?

You qualify by calendar year. If you qualify this year, it does not matter if two years later your income goes up. However, if you fund a Roth in the earlier part of the year and at year end you realize that you no longer qualify because of a raise, bonus or some other unexpect income... then you see your custodian and "unwind" the contribution. The qualification is done separately for each year. You can qualify, lose qualification, then qualify again over a series of years depending upon your income.