This is a dillema several of us have dealt with.
Depending on when you retire, in the year, you may not qualify for subsidies when you first retire. For example I retired in June - and had too much income already... but just signed up for ACA plans through CACovered for 2015.
There was a thread last year about how to show the *expected* income vs previous income.
ACA additional verification of income letter
And another thread this month on the same issue.
For the recently retired how do you resolve ACA wage inconsistency, big drop in MAGI
Some people just showed termination/end of employment letters. Others showed brokerage statements, and some wrote letters, pdf'd them, explaining their income.