Our Classipress install is heavily modified to include things like an AlertPay gateway and a custom drill-down through State/City and then categories (all done as sub-cats)
While that's great, the latest upgrade changes from WordPress categories to custom taxonomies, which means half the code needs to be changed. As we have a live site with 18000 ads our down-time needs to be minimal.
Does the Classipress licence allow you to install a test copy in a sub-domain?
We may also be asking for help in the mods section with the upgrade, so any early volunteers appreciated.