This seems to come up again and again with my customers making browser extensions: how do you start charging for a product that is currently free? It's especially important that it go well given that browser extensions are quite dependent on good reviews in the Chrome Web Store and some users will get pissed off and leave bad reviews.
In the discussion it would be wonderful if folks could share their real experiences with charging. Thanks!
I use Simplify extension for cleaner gmail look and the author of the extension recently changed its business model from free to paid. He mentioned his reasoning here https://on.simpl.fyi/p/choosing-a-business-model
I think it's fair to keep the existing users free because they have provided much more value -- initial validation, feedback, and they tolerated all of your initial bugs and shortcomings. Instead, tell them explicitly that you start asking for money, but you value them and want to keep them for free. And ask them for something small instead, like, tweet about your product or fill a survey.