David:
There are 3 possible solutions, all requiring either an investment in time or money.

• As suggested by Andrew earlier, you could purchase Dakota's Toolkit which will re-calculate based on historical transactions
• You can restore AR data from before Year-End to another folder, export the "Current Year" totals and import them into your current data as "Last Year"
• If you have GridView, you can create a report into the AR transaction file, filter by date and transaction type, group by Customer, send to Excel, format the file and import it as "Last Year"