Yes. It does not become a "verbal contract," but the concept is promissory estoppel. Look it up.

If you make a promise, and I rely on that promise, and I suffer financial loss, you're liable.

It's a good idea too. If you promise that a plan will be free forever, people should be able to rely on that promise. You might get beta tester, promoters, or whatever you want. If those people stop being useful to you, you can't just throw them on out on their bums. You're hosting their data, there's a cost to migrating that data away. If they've integrated with your APIs, it can be a high cost. If that what you'd like, promise "free" but not "free forever."

It almost never makes sense to sue -- it's just not worth it -- but a business which breaks a "free forever" promise is probably breaking the law, and in the era of the internet, is almost definitely breaking the law in some jurisdiction.