I think we still miss a universal, open source, Office suite conversion tool (Or maybe it is already out there and I haven't find it). For example, if you find an old .123 file, depending on the date, the best way will be to open it with SmartSuite 9.8, save it as newer .123 or as .xls. If you save it as .123 the only want to turn it into .ods is to use IBM Symphony 1.3 (?) to turn it into ods.
If you can not find SmartSuite (or older 1-2-3) you are doomed. So I think that an extendible open source converstion tool is required.