Conversion or export of old Freelance presentations (.pre) may be a little tricky nowadays, considering most programs no longer support the format.
The best we could find is that older versions of PowerPoint available around the time of Windows XP supposedly did support .pre files and could import them to the program. Once imported, it is likely that you should be able to save the presentation in one of the former PowerPoint formats (.pps for example), which you could then process in some of the later versions of the program to finally produce a .ppsx files. This is pretty much the only way how to achieve pre to ppsx conversion.