As moken alluded to in the comments, you can use the pywin32
library to open Excel and get access to lots of its internals, including being able to do a SaveAs.
Note that you'll install the library as pywin32
but you import it as win32com
(which is annoying...).
import win32com.client as win32
filename = r"C:\the\full\path\file.xlsx" # you do need the full path, it won't work with relative path
new_filename = filename[:-1] # new filename is the same as the old except for dropping the last character
xl = win32.DispatchEx("Excel.Application") # opens Excel
xl.DisplayAlerts = False # Makes the application invisible
wb = xl.Workbooks.Open(filename)
wb.SaveAs(new_filename, FileFormat=56)
wb.Close()
xl.Quit()
There are lots of other libraries you can use to achieve similar effects, but this one should do it.
Note that you may lose some functionality as you are going from a more-recent to an older version of Excel. This script replicates how it would work if you manually did all the clicks with your mouse in Excel itself, so any loss of functionality isn't related to the script, just to the "downgrade" itself.