If you want to use the same version of Studio and Runtime, you'll need to uninstall Studio and then re-install selecting the correct version of Office (you'll need to do this for Runtime). You can do this at a time you are planning to upgrade, as installing a new version on top of an old one (upgrading it) will give you the option as well.
In our project, we had the same issue in one of our developer's OpenSpan Studio. Then we went and checked all the references with another developer's OpenSpan Studio and added OpenSpan.Events, OpenSpan.Interactions, OpenSpan.Threading .. etc. and all worked fine thereafter.