The guys at the german website 3dchips.de have posted an article on how to mod your 9700Pro into a 9800 Pro pretty easy!
I guess you're all familiar with the mod from a 9500 to 9700. If not, check this page.
The new mod uses new optimized drivers for the 9700Pro with a driver path designed for the "Radeon 9800/Pro" series.
The guys have modded a normal 9700 Pro (Connect3D) in a 9800, and you'll see from the benchmarks that it worked ;)
You can find the whole story here. It includes a detailed report on how to do this mod, and benchmarks. However, the article is in german.
This is a babelfished translation of the page. If you guys want it in proper english or if you have any problems leave a comment. I can do that if there's enough interest.