Honestly, I'd wait and just get the classic later, assuming it's still around when you get it. It's not worth the $70-$195 BS Hassle you'll go through to upgrade the other - you won't see it later if you decide to sell it on eBay or Craigslist, and even if you need it now, getting a new classic will serve your needs down the road. Even though we've not had a 100 gig iPod in any line up (that I am aware of), we've had above that for at least 3 generations, so chances are it might be ignored.
If this were a 240 gig upgrade, I'd be more readily convinced of doing it: the most I've seen any iPods, new or past, without a HD replacement is 160, so from an investment standpoint, selling it later may bring back the money - and maybe more.
If you were doing more of a mod than that, I'd tell you to go for it - but even then I'd tell you to go for broke and get the 240, skipping over the 100 and 120 - You never think you'd need more until you have run out of space. 100 gigs is too small and (more importantly) too uncommon to be worth it.
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