Yes you can, in fact, during your Mandrake install, you even have that as an option should you choose it (advanced install).
You shouldn't hijack threads, at least not until an answer has been given to the original poster. Instead you'd be better off starting your own, this will not only give your more direct answers, but more answers all together since most people will view this thread to answer sneeze's question.
But yes, I've changed filesystems. Unless you are going from ext2 to ext3 or back, then you will lose your data (likely) so a backup is a must. Then it's just a matter of:
Each filesystem supported by your version of mkfs can be called by it's type, but read the man page for more info:
Kernel support is also a must, but as I mentioned, Mandrake definitely has it in the default kernel.