My thought was that the repair people would notify the owner that it must be turned off, and then do so.
I’m not 100% sure HVAC people are authorized to disconnect from the house current. They may have to call an electrician to disconnect the unit from the fuse box and then later to connect the new unit to the breakers — guess it depends how many palms need greasing (WHO SAID THAT? My account’s been compromised! 
). Either way, the power should be cut before any work is done. You’d think. 
You usually only have to flip a breaker switch, I would think. I used the word “disconnected” earlier but I mostly meant “circuit switched off”.
Yeah, anyone can access my breaker box. For that matter, there’s a giant “off” switch for the house, and anyone can access that, too.
The air handler (the thingie outside) is usually wired to a block with a doohickey with these massive contact blades. You pull the block, no power, safe to repair. Apologies for the lame terms I’ve used.
You may (or may not) be shocked 
to know how many people need to be called to bring things “up to code.” Some day, I’ll tell you about the rental house we lived in with the major sewer problem.
You’re a naughty girl. 
Yeah, that’d also be easy enough to switch back on, but I’d hope that it wouldn’t be something that the homeowner would do without thinking about it, like adjusting the thermostat might be.