Remove user role when membership expires

Hi everyone, I'm having some troubles creating my Wordpress website.  I have been working on this website for some time now. I'm currently working on a custom membership system and I need some help regarding user roles. I will try explain as best as I can and I really hope somebody can give me some detailed advice, I would be rather grateful.  When a visitor comes to my website and goes to the download page there are two categorys which are Free Items and Pro Items.  As the user is not registered they will not see an Add to cart button or the price tag on both category pages or product pages.  When a user first registers to my website they are granted with a Silver membership automatically which allows them to download items from the, "Free Items," category. This then hides the add to cart and price tag as the item is downloadable and free. There are another 2 Memberships which are Gold and Platinum.  As the user had only got a free membership if they go to the pro items category they will be able to buy the product unless they have one of the memberships specified above which will then remove the add to cart button and the price tag and shows a download button Instead. When the user purchases the membership Gold the role Customer is automatically changed to the role Gold Member for the settings to work within the download system which then gives them access to the pro items category. However the Gold Membership is active for 30 days and once it expires the role Gold member needs to be changed back to the role Customer automatically and that's what I'm struggling to do.  All advice and support is appreciated.