Can't turn off Multi-currency once it is turned on in your QBO plan??? How hard is it to convert historical postings from one rate to another at a historical average rate with an adjustment for any FX difference?? Not very hard. Intuit would rather keep you an expensive plan and not provide a simple option. 

Money gauging on part of Intuit when the nature of your business changes. Time to find better products from an organization that is not focused solely on their own bottom line.