You do not have to wait a year.
When you sign up they ask for your estimated income for the year that you will be covered and you provide that amount. Since that lower in retirement amount will be different from what the government has in their records for your income from tax returns, etc., it is likely that they will ask questions as to why your income changed and you'll need to respond explaining the change and providing whatever relevant supporting documentation you have. A number of people on this forum have gone through the process and I'm sure that they will weigh in.
Another approach some people use is to forgo the advance premiums credits and just pay the whole premium and get their subsidy later as part of their tax refund when they file their tax return. Those who take this option avoid having to provide their income and explain why it is so different, but if their income is really low they forgo subsidies for deductibles and copays.
Also, the income is not just AGI, but AGI + tax-exempt interest + non-taxable SS and some other adjustments.