I have never done a Debian install where I could not manually partition my hard drive if I so choose. Is it possible you missed the option?
You mention unstable in your subject and testing in your message?
There are two ways to go. Either should work fine.
The easiest way is to download an unstable/testing netinstall cd and re-install. You may need to type "linux26" at the boot prompt, without quotes, to get the 2.6 kernel. When at the boot prompt there is a help option. That will tell you exactly what to type to get the 2.6 kernel, if the 2.6 kernel is available. Depending on how you partitioned your drive this option may destroy your data. It's always recommended to backup your data before doing an install. I have a seperate home partition and have installed a few distros several times keeping my data. I of course did this because it was my play machine at the time and didn't have anything too important on it.
The second way is to upgrade. One of the great things about Debian is that you can totally upgrade your system without re-installing, if you want. The good part is that it actually works I wouls say >90% of the time.
I'm sure linuxquestions has a guide for upgrading. You can also check google.com. Be sure to read the release notes for the release you want to upgrade to!
Here's a link where others explain the upgrade path.
Short version. You need to upgrade one release at a time to be safe. Debian tests upgrading like this and has notes for upgrading in the release notes section.
I run unstable and just love it. I have had a few minor problems. They are usually fixed in a few days with an upgrade of the offending package. I have yet to loose any data or even the use of my machine although I had to re-configure my x-server once. Thankfully, I am not afraid of the cli. That was actually when I was running Sarge, before it became stable.
Good luck. You have already found the best resource on the web right here.