Make sure you clean your feet so they don't get infected. However, don't use really gentle handsoap, it will soften your calluses. A similar thing happens to fingertips when you play guitar, eventually the skin will toughen up. The first step is calluses, then the calluses go away and your left with tough skin that doesn't feel that much different than normal skin, it's odd. If you do grappling your feet will get tougher as well. Also, walk around your driveway in barefeet and around your house, your feet will get tougher. Also, I'd say if you have a bad open sore on your foot, just let it heal for a few days before you go back to muay thai.