What needs to be done to run a macro created in 1999 in Office 365 Excel correctly?

Created an extensive macro (Excel 1997) in 1999 consisting of several modules run together using "cntrl, shift, letters". Have been able to run it successfully ever since until recently, and it now gets bogged down and runs ever so slowly. The original spreadsheet that I've continued to use has a .xls file extension. 

I recently saved the spreadsheet with the .xlsm file extension but it makes no difference and will not recognize the quick access keys. Seems to have begun with the latest Windows 'update'. Will I have to rewrite the macro in VB or is there a simple fix?