HI, I have a Dell optiplex 390 MT that I'd like to swap into a empty XPS 620 case. Is that possible or won't it fit?
That is a bit trickier to say. Probably not the power/reset. But the USB and HDD and Power on LEDs are somewhat more likely. Audio is hit or miss, they don't always conform to standard there either.
They aren't vital to system function though. Rear USB, Rear Audio, and you can wire up your own powerswitch (or repurpose theirs) just by putting it across the standby power pin on the ATX connector and a ground. (Or just straight short it out so the computer turns on when power is applied, you can then plug it into a powerstrip)
If you really want them to work I can maybe do a deep dive, but unless you have wiring experience, and want to give it a try, there isn't much point.
On newer Dell's they like to use Power/Reset/PowerLED combos that are proprietary. And usually use a smaller pin pitch than is typical on front header I/O.