While the option for creating new long-lived access tokens is no longer available, we don't currently have a plan to disable existing long-lived access tokens. (If that changes, we will of course announce that ahead of time.) That being the case, you can continue using existing long-lived access token(s) without interruption, if you have any, but you won't be able to create more.
You can find more information on this migration here. Apps can still get long-term access by requesting "offline" access though, in which case the app receives a "refresh token" that can be used to retrieve new short-lived access tokens as needed, without further manual user intervention. You can find more information in the OAuth Guide and authorization documentation. There's a basic outline of processing this flow in this blog post which may serve as a useful example. If you will ever need to get new long-term authorizations, you would need to update to use this functionality instead.