Mac compatibility with Excel files

I have Office 365 for Mac with Excel for Mac 16.7. I have a full set of workbooks which were set up on Excel 2011. Is there any way to make them compatible? I can look at the files on the memory stick with the new program, but when I save the files to the computer (running Monterey) I am told that they cannot be opened once saved. Is there no way that these files can be opened, or will I be required to re-do the entire set of files on a new workbook?

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