You can’t do that

I concur with the views below. By removing those features it means anyone who inadvertently updates version 6 of the plugin will break the functionality of their site.

You have to release the premium version as a new plugin, and never allow version 6 to update to 7 as part of automatic updates.

And yes, the reputation of your plugin is based on the free functionality from it’s history. It would not get such a rating with what you offer in version 7.