Hi James,
As others have said, there are many different ways you can remove your existing AV and get SEP installed. To a certain extent it also depends on what sort of solution you are looking for. At the moment, here are your options:
1. Using the competitive removal tools on the CD - as you have seen, we have a number of scripts, and its very easy to add your own. Using some of the other utilties in the same location (such as the CompetitiveUninstallWrapper) you can actually get SEPM to push an entire package containing the uninstall of Sophos and the install of SEP, all in one.
2. We have a great little utility called "SEPPrep" which will ship on the RU6 CD which does a similar job, but actually searches through Add/Remove programs looking for AV/Security programs and removing them, then installs SEP - PM me if you want an early copy that you can use today.
3. Since we bought Altiris several years ago, we also have a cut down version of their software deployment tools that will first run an inventory and tell you what products you have installed (products, versions, etc.) then remove them and install SEP - since its based on a "proper" software delivery solution, it has really powerful things in like bandwidth management, local package distribution, etc. If you don't already have an enterprise lifecycle management tool, then this is a great route to take.
Not sure where you are located... but there maybe somebody closer than you think who could help you out with this stuff (all the folks in our PM team love helping new customers migrate!) so if you want a hand, feel free to send me a message here on Connect and we can probably hook you up!
hth