Dell usues a proprietary gang plug for the front I/O panel, and it usually has some extra features like multiple diagnostic lights, and Intake Air Temperature sensor. At the least you will get BIOS error messages, or maybe not boot at all. If you want to do this I would suggest moving the I/O cable and front panel module to the new case. Maybe in the floppy bay.
Also watch the screws in the cooler mounting. Dell often uses no standard threads there.