We're on the same page. First you stay up, then you improve with the time you bought.
When the load balancer for reddit broke once, we did't bother fixing it, we just replaced it with better (though untested) technology on the assumption it would work better. We figured it couldn't be any worse than it was, and we'd rather spend our limited time moving forward instead of treading water.
For most DB upgrades we did dual writing so we didn't have to have a break.