Turning on Multi-currency does not necessarily prevent you from creating back-dated transactions, canyesee.
Just make sure that all customer or supplier transactions must be recorded in the currency assigned to them to avoid errors.
After turning it on, you just need to manually change the transaction dates to the date before the feature was activated. For more details about how multi-currency works in QBO, please see these support articles:
I'll be around if you have other questions when recording transactions.