Whether you are changing systems or just want a fresh start with the latest version its worth trying the Sage Templates to upload data.
Once you have loaded Sage, click on Help, then About then Program Directory and you will find a folder called Import Templates, in that folder are Templates for:
Audit Trail Transactions
Fixed Assets Record
The templates are excel files and on the top row are comments on the content required. If you are already using Sage you can export the data, clean it and then re import it.
Basically as a minimum you will want to import outstanding customer and supplier transactions (using the audit trail), static data for customers, suppliers and nominal and unreconciled bank transactions. The opening trial balance can be entered using the Audit Trial import.
Use the Practice Company on Sage until you are confident you know your imports will work.
It takes a bit of time to get the imports correct but once its done it works really well.