WOW is that a big jump . . . Good news is that the upgrade process keeps getting better with each release. Bad news is when I did the upgrade from v6 to v8 it was a nightmare. The big thing was that they completely redesigned the forms / reports editor w/ version 7 so that after that upgrade I had to recreate all of my custom forms. I had people claim the old forms should upgrade, but mine didn't . . . I also remember getting an obscure error during the upgrade process, and Peachtree support finally had me remove every file that didn't have a .dat extension from the company data directory, then the upgrade completed. Which is probably why the forms didn't upgrade - even though I tried copying the .frm files back after the upgrade.
Can the upgrade be done? Sure. And I would argue that it is well overdue. There might be a few challenges along the way, but in the end it will be worth it. Obviously, I would start working with a copy of the company data to see how the upgrade process is going to go, etc. That way you can address any issues that might crop up without having the client dead in the water waiting for you :^)