Well as per your original post. It's not possible to redirect to the upgrade page. But you can edit the permission error to add a link to the upgrade page. This will mot effect any paid users / VIP's if you have their permissions set correctly.
To add a link go to ACP > Appearance > Search phrases > "text contains" I use: "You do not have permission" click the "do_not_have_permission" link. Then change to something link this. "You do not have permission to view this page or perform this action. <a href=/account/upgrades/>UPGRADE</a> your account to view this resource, image, or attachment."
Now your screen shot. You need to scroll down to "Forum permissions" and select "no" in the usergroup you want to restrict.
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