Starlink provides you with control over the activation/deactivation of the Starlink service. The key items to understand are that (1) your Starlink will activate within minutes after installation, and (2) you may cancel the service at any time, with no penalty or fee, in your user account. Given that both the activation and cancellation are up to you, and fully automated, there is no uncertainty regarding when the service will be available.
Below are the recommeded steps if you seek to ensure a seamless transition between internet providers.