When you copy the final totals spreadsheet from last year's workbook to this year's the formulae will include a reference to last year's workbook. Something like:
='[lastyear.xls]April'!D64
After you've copied the sheet you can do a Find and replace to remove the reference to last year's work book, i.e.
Find: [lastyear.xls]
Replace: {leave blank}
Look in: Formulae
Before running the replace make sure the worksheet names in both workbooks are identical otherwise you'll get a dialogue box for each cell where it can't find the source cell. Best to click the 'Replace' button first a couple of times to check it's working properly before hitting 'Replace All'.