We have a customised version of wordpress that we give out to our customers. It has a number of plugins and other changes including a custom plugin we have developed.
We have made changes to the install process to automatically configure various plugins and ours to save the user having to manually configure the plugin.
Basically we just give them a zip file and they just have to unzip, upload, change [url removed, login to view] file with database details, and pretty well it's fully configured.
Your job is to update to the very latest version of wordpress and all plugins as well as update our plugin in the zip file with the latest version.
The wordpress we are using is close to the latest version (2.8.x) and most plugins are close to the latest version.
So all you need to do is supply us two zip files for the two existing installations we will give you.
e.g. We have two separate zip files containing slightly different versions, you just have to update both and then supply us a zip file for both.
This is an easy job for someone who knows php and wordpress.