Why upgrade software?

A division of the company I work for, currently using ACT 2000, invited the ownrer of teh company to ACT training. He now wants to implment ACT in the other divisions of the company. However, he sees no point in upgrading the current users to the new version of the software. His question to me is why not install the older version of the software on the new users machines, or just run two different versions of the ACT software if licensing isn’t backwards compatable. HELP!
P.J.