I would like to change the default file format from .xls to .xlsx. Based on
suggestions in other threads, I've changed the template, Book.xlt, to
Book.xltx. Also, I deleted Personal.xls keeping only Personal.xlsm. I've
gone to Excel Options and changed the "Save files in this format" to .xlsx,
but that only seems to work for the current session. If I close Excel and
reopen it, the default is .xls.

The only good news is that changing the template and deleting Personal.xls
seems to have gotten rid of the compatibility mode issue, but it would be
nice to not have to select .xlsx every time I save a new file.

Any suggestions are greatly appreciated.