Income Increase - Budget

One and to amend the budget for income getting April of this year. When I went to do it, it goes back historical in adjust those months to the new number. This is for a salary increase as well as when CPP and EI are maxed out at some point during the year, I would like to adjust the income for that month going forward without impacting the historical data. So for instance, when I increase the income, my January, February and March numbers, change the income value as well and do not give a clear representation of what actually happened in those months based on actuals.