there was a change in library structure as multiple simscape products were merged into Simscape Electrical which prevents exporting back to old versions for components that utilize simscape electrical. There is a caveat though, that many components will still work if they were originally in the simscape power systems toolbox. Either way, a lot of manual work is required to do this.
However, you can also open newer versions of simulink in older version without having to post them. This will make it easier to manually tweak it till it works. Here is how to do that: (pasted from a different post)
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.
Remember to turn the preference "Do not load models created with a newer version of Simulink" back on afterwards.