It is only a ‘base year’ if you are at a CSS school that designates it that way. For FAFSA, you complete the form every year and the FA is based on the new FAFSA for the next year. For FAFSA 2019-20, the tax year 2017 will be used and as you said, it will be 10% of your salary for 6 months higher, plus $10k for vacation pay, than 2018 will be. It really won’t make a difference if you are talking $100k in salary or more.
(I thought the middle class tuition help in California was for incomes at about $100k for two parent families?)
A few schools do set the FA in the ‘base year’ as you say, and then guarantee to meet that level for 4 years. Very few schools do that. Those schools use the CSS and you can explain that your 2017 income is $10k higher, plus that your income will be 90% of former levels. For FAFSA schools, you have to answer the questions as asked (what IS your income, not what WILL it be) but you can send in an appeal to each school with a change of circumstances, that you have retired and new income is $XXX.
But again, if your salary/pension is above Pell levels, you won’t get much relief.