Version compatibility issues

My colleagues and I have been plugging away on an ongoing project that has spanned over several months. During that time, we have updated to the latest Captivate versions as they have been released. With 12.6, we have run into an issue. We are now unable to open any project or simulation that was created in a previous version (ex. 12.4). This is a huge problem as we plan on continually updating these courses as changes occur within our organization, products, and services. Has anyone else encountered this issue? What is the fix?