"How to reclassify sales for past 3 years? Sales have been charged to WRONG account (Chart of Accounts) so showing incorrectly on Profit and Loss statement. QBO provides simple method to reclassify expenses. I can change individual journal entries of select bunch and mass change them. I can't do same for income accounts and need advice."
It depends on how the sales revenues were recorded.
If the Invoices or Sales Receipts were created to record the sales transactions and they are Services and Non-Inventory types (except Inventory type), you can update the historical transactions by changing the Income Account on the product/service information update page. All you need to do is select new income account you want to use for that item, you will see "Also update this in account in historical transactions" and select that checkbox and save. It will reclassify to a new account for all the historical transactions.