Yes, 1500MTU traffic will still flow on a 9k network no problem. This should not cause any interruption, but you should consider placing your host in maintenance mode to test vmkping commands (i.e. vmkping -d -s 1472 <ip of another ESXi host>). You should also consider unchecking Host Monitoring from Edit Settings on the cluster to prevent any false isolation events during your maintenance. Also be sure to review portgroups and vSwitches as it can be set in both places. As of 5.x this can be done purely in the GUI.
PS - Just curious why you are leaving the fast lane? Have you experienced issues? If anything I would keep jumbo frames active for any vmkernel interfaces connected to network storage. Management and VMs I don't typically jumboframe anyway. Best of luck!