Hi,
I think its really important to understand your business requirements for a project like this before diving into the detail. (Rob N - couldn't disagree more with you on this one). Lots of companies don't take the time to consider their business requirements when doing something like an upgrade.
The BRD will probably be quite short but that is often the case in infrastructure projects. Also consider non-functionals at the same time e.g. response times.
Looking at your scenario above I can see some requirements already:
1. The system shall fit within our current desktop architecture
2. the system shall fit within our current server architecture
3. the system shall be supported by the vendor (can get much more detail here on when support required, response times, how its supported e.g. phone, on site, etc)
4. The system shall provide like for like functionality (you'll need to define what the functionality is at some point)
5. The system shall support existing business processes (can go into more detail based on the processes you have defined.)
etc.
Have a go, you'll be surprised at what this exercise will throw up. Particularly when you speak to the business and get their opinion, you may find that some of the assumptions that everyone is making are not actually true. e.g. a like for like conversion. Bet the business has identified some functionality they need that no one has bothered to ask them about.
Kimbo