That'd be a good problem to have
I hope we're faced with that issue sooner rather than later.
My own gut feel is that the current roadsters, particular the 08's, are likely to be left behind rather quickly, unfortunately, as the focus becomes solidly on the new platform for the Model S and subsequent family of cars. Even without the completely new platform we're already seeing the 2010's get substantial new abilities that may not ever be realistically possible to upgrade the 2008's, even as paid upgrades.
I think Tesla will do more than most auto manufacturers in this area, and I guess I do hold up a hope still that they'll decide it's better to pull all the 08's into as close a conformance with the 09, er, 10's+ as possible to eliminate stocking and service overhead, but I'm afraid that'll just prove too costly to support. Don't know which way that'll go, how many spares of the original components they have, etc. but it'll be costly either way
Battery box and associated PEM improvements are really going to depend on how many roadsters they manage to sell with compatible configurations (ie, 08->10 would take a lot of work to change to) before they change they whole approach with the model S "sled" platform.
If they sell enough, it's conceivable there may be an aftermarket if not corporate upgrade capability. Hope so. As always though, it'll come down to what makes sense economically rather than what's theoretically possible, except for a few driven, genius type tinkerers (gotta love 'em - they help push the boundaries).
I can see replacement packs for the roadsters weighing less, and even potentially having the weight lower, but expect they'd have to stay the same volume and strength/rigidity. Yeah balance would be different, and may take some tuning, but it'd be better. The pack would have to conform to the interface (voltage, power specs, interconnects) expected by the PEM or it becomes a bigger change. That's what we are seeing already with the 10's. It's a cascade of necessary changes, throughout the electrical/cooling system. Can't replace just one subcomponent without changing it all. I'm sure that extends all the way to the firmware and fleet of controllers/sensors too. Without stable module/interface boundaries, it'd be tough for anyone but Tesla to offer updated components.
Anyway, I'm rambling now, as I'm want to do regardless, but especially when giving opinion rather than fact