Big Sur is relatively irrelevant :-) The concern is that you are using a more current edition of Mac Office software which no longer provides the necessary filters for the deprecated variety of .doc file type produced by the Office 98 generation of Word.
One option is to open the files using a current Windows edition of Word, then converting them to the .docx file type. You may also be able to use the free LibreOffice software to open them & Save As in the .docx file type.
If there are too many to deal with individually you may be able to use an online conversion service [I haven't used it but I understand Zamzar to be pretty good] or someone here may be able to provide an automated batch conversion procedure.
Either way there may be some reformatting required depending on the nature of the content & its document structure.