unless they are willing to continue with sage I wouldnt do it.  You could lose information when importing back. If they are going to stay with sage I would suggest have them do import for you.  

As I understand this issue you are converting from QuickBooks to Sage because the company has made the decision to move to the "better" program. (Personal bias!) There use to be a conversion program available and the Sage Support Team will probably still have it and they can help. If it is like most conversion programs the transactions from only the current year and one year prior will be converted. 

The alternative would be to keep your QB file running for history and start a new Sage file. You can transfer/import three lists from QB into Sage: Chart of Accounts; Customers; and Vendors. You cannot import transaction. However, there are Sage Consultants who can migrate the transactions.

Hope this helps