When you start subscribing to Reason+, you still own everything you owned prior to moving to Reason+. Whatever you bought, you’ll have access to, of course.
Check the Rack Extension page in the Companion app to see and handle your available Rack Extensions.
3rd party ReFills are managed just like before, i.e. you save them in a folder on your computer and have a shortcut to the folder in Reason's browser, as described in this article.