If I understand your question correctly, you have a file (same file name I assume) that either has the old or new file format and fails because your flat file source has the old (10 column) data file schema only? If this is the case, I would create a boolean variable and call it something like isOldFormat. I would then use a script task in your control flow to determine if it has 10 or 15 columns. The pseudocode would be something like this:
1)Open flat file 2) Count the columns based on your delimiter 3) Condition Statment:
If columns.Count = 10
isOldFormat = True
Else If columns.Count = 15
isOldFormat = False
Else
throw error
Then I would create another data flow that would have the new file format schema (now basically you have two data flows-one with your old file format and one with the new one).
After this step, you would then drag precedence constraint from your script task to the newly created data flow and one to the old data flow. By double clicking on your predence constraint, you would then set the evaluator operation property to Expression and type in the Expression box @isOldFormat == true for the constraint going to the data flow that contains the old flat file source and @isOldFormat == false for the other data flow. What this will do will only execute one or the other data flow based on variable that is set in your script task.
Hope this helps.