Wading into this a year later. New to the game and just tried going from 9 - 30 days on an existing world.
The spoil rate needs to change to make sense, so I wouldn't feel odd about changing it. I am watching food loose days on it's shelf life in under an hour of in game time. That means the spoil rate is somehow not properly scaled when the month length is.
Edit:
So I changed spoil rate to the recommend 50% (0.5) - and it's better, but watching food on my shelves; we're losing 1 day of shelf life for every 30 minutes of shelf life still - going to see if I can get it to match the time speed and maybe there's a formula we can work out for players.