What happens to existing transactions when changing to multi-currency and changing the base currency? will pre-exiting transactions remain in original currency?

Afaik, once you have registered one transaction, you are not allowed to change your home currency any longer. You need to open a new QBO account, turn the multi currency on, then change your home currency prior to do anything else.

https://quickbooks.grsm.io/Canada

https://quickbooks.grsm.io/Canada-fr

Utilize the 3rd party tool to migrate between QBO accounts.

https://transactionpro.grsm.io/qbo