|"the new motherboard takes a 20-8pin"|
20-8pin? That sounds like the main ATX power connector?
If all the front panel wiring (power, reset, HDD activity, power LED, USB, audio, etc) comes into a single plug, you're going to have to carefully remove each wire/plug from the plastic connector, then individually connect them to the front panel header on the board. If you look closely at the plastic connector, you'll see the small metal plugs are locked in place by a plastic tab. If you carefully pry up on the tab with the tip of a knife, the plug should pull out of the connector easily. You'll end up with something like this:
Then you'll need to figure out where to connect each one of them to the new board.