I think it's going to be similar to the 10.0 -> 10.1 installation... you can either upgrade or wipe the hard drive and perform a clean install.
On the Jaguar "more features" page, it lists a "Clean Install Option" down towards the bottom -- i think this is a la OS 9 where you can perform a clean install of the system without having to reformat, and your old files and what-not will be placed in a "System folder (old)" folder or something like that.