Purchase of an existing property - Support with financing

Does changing the tax class for the tax return still have any effect? My parental allowance was calculated based on the gross income...

Parental allowance is based on the average net income of the last 12 months. This is calculated from the monthly gross income minus taxes (where the tax class for the tax return does have an effect), statutory social security contributions, and 1/12 of the standard deduction for income-related expenses.
In this respect, changing the tax class for the tax return did and does have an effect, yes.