Surprisingly, they both appear to accept Micro-BTX motherboards, so I believe you could put the motherboard/CPU/RAM/I/O Shield of an Optiplex 780 into the case of an Optiplex 620.
I'm having poblems finding i/o shield from a dell optiplex 780 mt. Any ideas or thoughts to who or where I could get one? I've tried Ebay. Thanks for your help.