To stay afloat, I want to start charging my early adopters, who have been using my product for free. What is the best way to approach this problem?

Just do it!

I’m a firm believer in charging for your product from day 1.  No matter how un-polished you think your product is, the only way to tell if people derive value from what you provide is to attach a dollar amount and charge your customer.  It’s difficult to make the jump from a free product to a paid product, so if possible charge from day 1 and avoid the problem altogether.  If you’re already giving your product away for free, then use this to your advantage by making an educated estimate as to the value you’re providing your customers and use this data to set your pricing as you see best.