That doesn't sound good. Will this cause a running issue or just mean that when the oil is drained it won't drain everything. Is there a better alternative that doesn't involve replacing the cases?
I installed a Helicoil in a '97 KX250's oil drain several years ago...very simple job, about 15mins.
All you have to do is measure the depth, if I remember correctly, 10mm was good...then you want call Helicoil and order an M10 x 1.25 x 10mm stainless steel helicoil, and get a small bottle of Loctite 266, you'll be putting that on the outer threads of the helicoil just before you screw it into the case, then it will never move.
Time-Serts are another good way to go...but since the drain plug is such a low torque bolt, it's a bit overkill, and quite a bit more money.
I ran my '97 for 4 years with the helicoil...sold it to a buddy, who rode it for another 5 years without any issues. The key is to remember not to crank down on the bolt, just snug it up and safety wire it.