If expanding your network does not cause you to overlap into other networkspace in your current routing table you should not have any issues because you don't have any devices outside the current scope. If you however move the gateway outside of the boundary of the current /28 you will temporarily lose routing.
So if you have to move your local router outside of the bounds of the old /28 range I would first config the switches to the new subnetsize and gateway and afterwards change your router so connectivity is restored.
However adding a separate VLAN for management is better because switching itself won't stop working even when you flip from one mgmt network to another. I always have a separate VLAN for switches and AP's.