Backward compatibility in client software

Correct, that pain point existed since the begining and you should see lot of posts on the same topic already in the community. You just have to plan the upgrade process to overcome this.

Usually for windows application, we can always install two thick client versions of windows application on the same PC which lets you access the environments independently or launch the applications directly using the web URL or through the oneclick setup.

For excel add-in, we need to make the two different software's readily available for users so the can install or re-install them as per their testing and production usage needs. Alternatively for users should have the ability to use "client update" option present within windows application to sync up their excel add-in as per their testing or production needs. 

Hope that helps