[Centos] Upgrading Heavily Modified RH9 Machines

Hello,

I have several heavily modified RH9 machines (custom kernels, reiser FS, etc.) 
Just wondering if CentOS could do an upgrade install on machines such as 
this? I know it's always better to do a clean install, but these machines 
have been tweaked a lot over the last year and are running perfectly (a lot 
of time and effort went into this)... an upgrade install would be great. My 
main concern is the filesystem.

Thanks,

Brad