For each month of the year, I have a budget column with sales and cost data
(e.g. in column B) and then next to that I have a column where the actual
data goes when the data is available.

                    Col B                 Col C               Col D            
 Col E
                          1m Budget        1m Actual       2m Budget     2m
Actual
               
Row7 Sales    $100           $110                  $105       
               
Row9 Labor Cost    $40             $55                   $60       

I want to be set up a formulae to be able to sum the data for all 12 months
for each row using the budget data, but as the actual data gets filled in and
becomes available I want the actual data to be used instead of the budget
data for that month so the total year forecast becomes a combination of
actual data and budget.

For example the original forecast for sales would have been 100 (month 1
budget) + 105 (month 2 budget) = 205 but now the revised sales forecast is
110 (actual month1) + 105 (month 2 budget) = 215. The data runs from column B
through to Y.

Do I use a sumif if a value is in the actual columns and how does this get
set up to check the actuals automatically and sum them instead of the budget
columns to the left if they exist.

Many Thanks!