I just tried to reproduce it and got a big warning message that says that data will be lost. You can disable this message (per file basis), if for some reason

I just tried to reproduce it and got a big warning message that says that data will be lost. You can disable this message (per file basis), if for some reason you dont want to see it. As far as I remember this message existed since Excel 2007 (which introduced .xlsx format), so the guy is simply lying.