Well you could power up the deck and eject the CD. That's about the only way to get the CD out without damaging the deck.
It SHOULD be pretty easy to pop your new HU out and temporarily reconnect the old one to get the CD out. Did they give you the removal keys your new HU came with?
If not there are other ways to power the deck up, but you need a 12V source and 3 alligator clip jumpers.
