help translating open source to NPOStaff

I work at a small-medium sized non-profit. At our main site we have a windows network consisting of about 20 computers. The network has been setup as a peer-to-peer network. Because of this administration is decentralized and it is difficult to do tasks such as backups and sharing resources. I have decided to implement a samba server as the primary domain controller and move from a workgroup to a domain. So far so good. The server is up and running, no problems there. I have been slowly moving computers onto the domain one-at-a-time. This is mostly because I am not a Linux or Samba guru and I wanted to make sure everything was working fine. Now I have two computers to move to the domain, but I fear I will receive resistance from their operators. I am the only IT person here and no one else really knows too much when it comes to anything having to do with technology. I am wondering how I can approach these two individuals in a way that will either reduce resistance or will even make them want to be switched to the domain. They are very concerned with security on their computer. They have financial records and do not want anyone else getting close to them, for this reason they do their own backups, and would most likely continue to do so once their computers are on the domain (I think the mentioning of automated backups may scare them more then comfort them). Any suggestions anyone has would be greatly appreciated.