There are several ways to set your default and switch between the two.

1. As suggested by Gord Dibben, use regserver to set your default version
2. Right click, open with, and browse to the version of excel.exe you want to use as a default. The selected Excel version will register itself.

These work fine if you just want to set your default. But painstaking to switch back and forth.

3. Fast switching

a) Select your default using 1 or 2 above.
b) Create a batch file with the command
"C:\PathToAlternateVersion\excel.exe" %1
c) Right click an excel file, select open with, browse to the batch file

Now if you double click an excel file it will open with the default you selected. If you right click and open with you will have a choice of versions.