Indeed, T07 is correct. I upgraded a Jukebox 3 in a similar manner. It's straightforward and very satisfying!
Contrary to fappar's assertions, the calculations works out slightly in favor of upgrading yourself. The price of a new Zen is about $270, a Fujitsu MHT 60GB from newegg.com is about $178, and -- here is what fappar missed -- you can sell the 20GB for more than the remaining difference, if you don't absolutely suck as an eBay seller, of course
. You end up saving money if you upgrade yourself, but definitely not time or energy.
For the moment, however, there is another difference between buying a new Creative Zen 60GB and upgrading to 60GB yourself: the firmware is hilariously better on the Creative version. As far as I know, the highest version for a 20GB USB 2.0 Zen is 1.01.01, whereas the Creative-branded 60GB comes with firmware 1.11.01. You get things like dynamically scrolling track and album names in all views, a new "Modern" Now Playing skin, clock screensaver, etc. Not to mention twice the track limit.
I have no idea what the latest firmware for the USB 1.1/Firewire looks like; it's currently at 1.12.02. Perhaps Austonia can help with that.