I know there's button on Dell 35xx that I can change the stacking status during the reboot. But on Dell 6248 I didn't find such function, nor did I find the way to disable in the manual.
In Dell 6248's manual, it did mention when all Stacking LEDs are off, "The unit is operating as a standalone switch." So there must be a way to set it, but so far I didn't find it