Shadow, at 14, might be having problems OTHER than Clayton your new cat. She is considered old now and may have problems reaching a litter box that is too far away. The kindest things you can do for her are (1) a vet visit for a senior's profile to make sure that she is healthy and (2) provide her with as much comfort as you can and yes, that may include moving her litter box to the upper floor where she feels the most safe.