A DC motor specified to 15V nominal will rotate faster at 24V, and excessive speed wears the brushes out faster. Is it a problem? Maybe, maybe not.
Another issue is, if the motor is rated at 2A nominal, if you just plug the motor in to 24V when it's standstill, it will take significantly higher current (stall current), maybe 10A, maybe 20A. The SMPS would probably just enter short-circuit protection mode (hickup protection for example), so the motor would not turn.
Solution to this is active current limiting (usually done as a PWM controller driven by a current sense signal), but then it becomes a motor controller design job. But if you do that, then you can also limit the maximum duty cycle, so 24V will not be problem either as you can limit the motor speed.