another good reason not to migrate to Win10. Mind you there is always "Sendkeys" in VBA, maybe that would solve it.

I remember an Apple evangelist who was complaining he could not use most of his old software in OS X. Then they migrated to Intel processors. And they still say Macs don't get viruses.

any files compatible in both like .csv files is certainly the way to go. The problemo is that .csv only does one sheet at a time. But in VBA that is just a question of how long it takes to process. The point is .csv files are so basic and just data, which means there is less chance for incompatibility. And compatibility is relative. To complexity, I found in 45 years of compatibility.

One has to assume some customers insist on one format, others the other. But if you can hop between the two I would guess .csv would do it. My take would be (in a macro) to import into a sheet(s) and then slave the data (eg in sheet1 "=Sheet2!A1") to where the clever cell formulae reside. No fancy programming once the data are in the spreadsheet. It would duplicate the number of sheets but they can be hidden. I do it across spreadsheet files just so I don't have to re-invent the wheel. The original files are largely untouched but carry current data.