Hello, I've been having a problem with Excel losing formatting data.

I copy sheets to a monthly workbook (each sheet is one invoice)

After a while, sheets in the monthly workbook lose their formatting (ie. borders disappear, fonts and font positioning resets to default) this seems to happen only in parts of the sheet where I have changed values then copied the sheet.

The sheets which I copy from I reuse by typing over, then saveas, then copy to the monthly workbook. All file formats are xls ('97-'03 Compatability).

What's going on?

Would I solve the problem by converting them all to .xlsx?

Any help/response appreciated.