you could do that, & it SHOULD work with minimal problems, but..........
since this was a major version number release (9.x to 10.x) you might run into library probelms with some apps. it's usually recommended to do a fresh install (preserving /home if you want) with major version number changes.
if you're trying to avoid downloading the ISO's due to limited internet access (IE: slow dial up) then it's still gonna take forever because you will be pulling all the packages from an ftp site.
NOTE: before doing any type of major upgrade, it's always advised that you make backups of anything that you don't want to lose.
if you still want to attempt it, go to Easy URPMI
& set up a "main" & "contrib" source (if you don't have them already..........if you do have them already, then proceed to next step.) you need to add sources in terminal as root.
.......remove all other sources (CD's, Cooker, PLF, Jpackage, etc.). the easiest way to do this is in terminal as root type the following command........... urpmi.removemedia -a (hit enter). that will delete all of your sources. then get the above Easy URPMI sources. (if you already had those sources, after removing your other sources do........ urpmi.update -a (hit enter) to update them.)
now, you have "main" & contrib" set up, all other sources are gone. we need to kill X before you start the upgrade. in terminal as root, type........... init 3 (hit enter). that will kill X & drop you at a command prompt. login & su to root.
now, for the grand finale. you need to activate internet access. do that with whatever command you use for your type of service. (IE: adsl= adsl-start, etc.) (to eliminate this step, don't disconnect from the internet when you do the init 3 step from above. that will only kill X, but leave your internet access active.) and finally, issue the following command........ urpmi --auto-select (hit enter). sit back & watch & wait. if all goes well, it should find all packges that need upgrading from your 2 sources, along with all dependencies. when it's finished, cross your fingers, hold your breath, & type........... reboot (hit enter).