To store mics, you have to put it together to lots of silica gel to remove possible moisture inside the capsule. Normally, if stored properly, the mic response will not be affected. Storing it would mean putting in in a place where the humidity and heat factors are not that great to affect the mic's performance. Try re-using them to "exercise" them a bit and see if there will be some improvement in their performance. Also, put some silica gel into their storage boxes. FWIW