Technically, you cannot change a start date. The start date is an arbitrary marker point for you to organize your accounts. It has no effect on QuickBooks Desktop accounts or transactions. Changing the start date in the EasyStep Interview will not change the dates on transactions that are already entered. When you originally completed the EasyStep Interview, you probably chose a start date, and then any account balances that may have existed before that date, you entered as opening balances. When you start using QuickBooks and entering transactions, these transactions are added to or subtracted from this opening balance.
Once you have started entering transactions, if you decide you want to change your company's "start date", you will need to adjust the opening balances for your accounts and then enter transactions that make up that adjustment with new dates (prior to the "old" start date).
For more information, see edit an opening balance and enter opening balances for customers and vendors.