Flat File connection Manager can be manually modified to add a new column or remove existing columns, there is no need to Rest Columns which removes the customization done on existing columns.
Be careful and make sure to insert new column at the appropriate order to avoid column mismatch.
SSIS Does not read column names and read the data in same order but just pulls the data and assigns to the columns we have created.
I had a package which reads the data from Intacct and pushes into Text file and this data is processed to SQL Server. I had to add new column! I just added new column at the end in my Intacct column list expecting new column will be the last one in my text file. New column was added in a different position which created data discrepancy.