Adam,

I am not sure about changing licenses that you have already purchased.
This FAQ from M$ listed a question/answer:

Q. If my business needs change, can I switch between user CALs and device CALs?

A.

If you have Software Assurance for your SBS 2003 R2 CALs, you can only switch between CAL types when you renew your Software Assurance contract. If you do not have Software Assurance for your SBS 2003 R2 CALs, then you cannot switch between license types.

http://www.microsoft.com/WindowsServer2003/sbs/evaluation/faq/licensing.mspx

You are probably out of luck.

The original installation of the SBS allows for selection of either a user or device CAL or a mix of both for the first 5 licenses. After that each pack of CALs you can choose which license type but I think that you choice all one or the other, you can not mix and match. In the new server 2008 CAL structure, each type has a different price point with user CAL being slightly cheaper then the device CAL. I am no expert in this particular area, just a basic knowledge of the issue. I work in a hospital system where the IT manages the domain and network.

If you can not switch to the user CAL then just keep the device CAL. A mixed CAL environment is probably more trouble than it is worth.

I would mention to anyone reading this post that most AC users would be better off with the User CAL.

Geoff