By turn all old aricles 'Pro' you have just limited your user experience and will lose users
In an attempt to monetize your product you just pushed your users away. You're best bet is to monetize through advertisement and have as open a community attracting as many users as possible. By instead trying to monetize the content from the user rather than advertising to them, you will limit how often an article is viewed, lower the amount you can charge for advertising, frustrate the users and encourage them to get their information somewhere else, and would encourage a rival page that doesn't suffer from the audacity of charging their users for what used to be free content.
You don't have to believe me. I know you track your users. See how many less articles are loaded over the next month or the next quarter. See how much less your advertisers are going to want to pay for such limited exposer. See your business model crumble by one incredible stupid mistake.
I hope you don't take a full quarter to figure this out. It might start a spiraling trend that you won't be able to get out of. I wouldn't want to be the one how came up with this idea....