New client who i have done 1 quarter vat for. he was with xero before. can i import old data without affecting the work ive done on quickbooks now?

Let me answer your question about importing data, balanced-accy.

A new import using the standard data migration process will overwrite the information that you've already added. 

You'll want to import the old data using a CSV (Comma Separated Values) or XLS/XLSX file so it will not affect the existing data. Some of the information that you can import are the list of your customers, suppliers, accounts, and products and services. 

Let me share these articles so you can follow the steps and import them in:

• Import products and services from Excel 
• Move your chart of accounts to QuickBooks Online 
• Manually upload transactions into QuickBooks Online 
• Move your lists to QuickBooks Online 
• Common questions about importing data to QuickBooks Online.

Let me know if you need more help when reviewing the data after the import process.