Eventhough Asterisk is good for smaller applications, and if you can get things setup it can be good for bigger applications, but I would suggest keeping the new PBX.
Asterisk is now Trixbox, and it sounds like the IT guy just wants a new project. I would have to ask how many extensions there are, but I don't believe it would be to the benefit of your company to trash a nearly new PBX and install something like an Asterisk in it's place.
Is your current PBX a true PBX or a VoIP PBX?
