It's nice to have you here in the Community. I'm here to help import your customer account numbers in QuickBooks Online (QBO).
In order for me provide you with the best resolution, can you please verify what customer account number you're referring to? Is it a Bank Account Number or the number you've assigned to your customers? Additionally, when you say "old system", do you mean your previous QBO account?
Once I know this, I'll be able to work towards getting you the fastest resolution. You can leave a comment below by clicking the Reply (green) button to add more details.
Looking forward to your response and providing further help. Enjoy the rest of the day!