You have to purchase a 3rd party service to convert data from 2019 version to 2018. Some data may be missing during the conversion due to the lack of feature in older version. I would prefer to upgrade your 2019 data on 2020 version.
QB Desktop has more features than QB Online. You may purchase QBD 2020 with a one time license thru a partner/reseller with lower price. You may deploy a private cloud on your premise or put the data on a 3rd cloud hosting service. Otherwise, you can migrate to QB Online.
https://quickbooks.grsm.io/Canada
https://quickbooks.grsm.io/Canada-fr
