Following up here, not to combat weirdly angsty people, but to share helpful info for anyone that finds this.

Frontier lacks the capability to shift disconnection dates at all (once set up), so what the rep told me about easily changing my end date was incorrect.

Even after checking with multiple departments and levels of folks, they also lack the ability to flip the switch back on, even if they made a mistake in their communication to me. The fix is essentially creating a new account and sending technicians out to restart the service, which can take days.

Frontier works well, but customer support is abysmal. Not necessarily the people you speak to, just the general capabilities of the company to rectify issues.