Find out their reasons why they think they need to go VoIP and make sure they are valid. Most people think it is going to be a lot cheaper as you will get rid of all your voice lines. That is not so in most cases and those you do get rid of you have to be replaced by building up your network. The big savings begin to shrink from that point of view.
One of the biggest bangs for the buck, if you have multiple locations, is that you can serve them all off one switch/PBX/Call Manager instead of multiple, which saves money on maintenance costs, one upgrade for all instead of multiple for each system and there are some savings in reducing your lines.
The biggest negative, as KCFL ... stated, if your network goes down, you lose everything. Also up front costs are relatively huge, unless you are dealing with a hybrid PBX or similar.
If you have a strong, robust network, with little to no downtime and a good network team to back you up, there should be no issues going to VoIP.
In most cases I have come across, the phone guy takes care of lines, cabling, the desk phones, moves etc, all things that most network guys do not want to do, so phone guys stay. you will end up doing most of the programming of phones, voice mail etc too. Most Avaya/Nortel PBX can be easily upgraded to VoIP saving a bunch of money on a new system. You can put new phones on all the desks, but most of the stuff you are familiar with is still in the closet.