How locked down are they? Should I expect to see a hacker conference talk "how i revived a weather satellite"?

Maybe once they're turned off they're irrecoverable?

There have been numerous issues in the past with 'dead' satellites waking back up and activating transmitters again, generally making a mess of the spectrum. Some satellites have pyro-fuses on the power lines from the solar arrays that are fired during passivation to make absolutely sure that the sat doesn't ever recharge and wakeup.

It's not impossible

https://www.youtube.com/watch?v=HT2i7mFpFxM

I agree they may want to reuse the spectrum, but I doubt it would be for a new satellite. At least not the stuff in the lower VHF band, around 137 MHz which is awfully close to the airband reservation used for VDL Mode 2 - around 135-137 MHz, IIRC. VDLM2 is in many ways a more modern ACARS.

They'd more likely use higher bands on newer satellites to get more throughput. The GOES birds transmit up around 1.7GHz, afaik and likely higher as well.

You have to have an antenna pointing at the satellite though. That's an additional layer of selectivity.

Makes total sense, thank you