I assume each case has a different lock, so you probably won't be able to replace the key without replacing the locks.
They're all keyed the same.
Based on youtube lockpicker viewings Id be amazed if there is anything complex going on with these locks. Its probably a pretty standard type of key that a decent locksmith could help you with if contacting Yani customer service doesnt get you what you need.
https://www.worthpoint.com/worthopedia/yanagisawa-tenor-saxophone-hard-case-520379445
These dont look anything special...
They aren't anything special. Unfortunately, Yani's website points me to Conn-Selmer for customer service. Conn-Selmer can't help as these are "imports" and they don't get extras.
Erik, if you are truly OCD about this, then you should recognize that a replacement key will NOT satisfy your innate urge to have ORIGINAL case candy.
I usually throw away case keys. They are great for locking your case, then getting lost at the worst possible moment. If anyone wanted to steal something from your case, they would just walk away with the entire case and horn. Seriously.
It's more a matter of keeping the kids out of the case, not keeping someone from walking off with it. They're at the ages where they get curious about my instrument collection and start fiddling. I'm a huge proponent of kids learning music, but not at the expense of my collection while I'm not home. As far as my OCD goes, buying replacements through SKB for my Fender bass case was enough to satisfy me.

The kicker is, if I can remember where I put my Yani keys, I'll have my original Fender keys, too. I'll probably find a bunch of stuff I've misplaced over the years all in the same spot...
If anyone has keys they don't want (I've read a few times that people throw them away), I'll happily pay for them and the shipping to me.