Simulink has a preference, in the Model File section, called "Do not load models created with a newer version of Simulink". You can temporarily turn this off rather than edit the model files to change version numbers.
If the model uses only basic blocks then it might be possible to load it in R2017b, but it could also crash MATLAB. As others have commented, using the "Export to Previous Version" feature is the supported way to take a model from a later release to an earlier one.
Remember to turn the preference "Do not load models created with a newer version of Simulink" back on afterwards.