@isofos 

Should you need to merge the historical data, it would be affect your 2020 data inevitably. e.g you may have to revise all opening balance. You may need to complete the data migration in 2 steps:

1. Firstly, open a trial account of QB Online to convert your QB Desktop file

https://quickbooks.grsm.io/South-Africa

2. Secondly, utilize a migration tool

https://transactionpro.grsm.io/qbo