Depends. Brammo hasn't made any hard commitments, but they've indicated that they'd like to make battery upgrades a possibility. A drop-in replacement would be fantastic; but keep in mind that the batteries will be expensive and heavy.
Here are a couple more examples:
Zero sells an adapter
to allow the 2010 X and MX bikes to use the 2011 pack. They don't have an adapter for the S and DS bikes which use fixed battery packs and would require substantially more disassembly, nor do they have an adapter for the 2012 packs (which are about 50% higher capacity).
A third party sells an 8.0 Ah pack
as an upgrade for the 2000-2006 Honda Insights and Civic Hybrids. Insight uses a 144V 6.5Ah pack (at least until 2005), Civic Hybrid uses a 6.0 Ah pack. Capacity isn't a huge concern for a hybrid, but the new packs supposedly can put out a little more power, tolerate a little higher charge rate, and run a little cooler. Cost is a bit higher for the new pack ($1800 + shipping) than a refurbished pack ($1000 + shipping).
Historically, battery technology has improved slowly enough that an upgrade may be pretty minimal. In the case of the Insight, the new packs are 25-30% higher capacity and power from the original packs 10 years ago. It's pretty attractive if you are replacing the old pack anyhow - but maybe not something you would pay to upgrade early.
Additionally, most battery-powered electronic equipment is pretty well outdated by the time significant battery improvements have been made, reducing the payback on designing an upgraded drop-in pack. This will probably change as we use packs for transport - a ten year old car may be a little rough on the interior and exterior, but generally an electric powertrain will still be working well.
The big battery increases we've seen recently (Enertia 3.1 kwh to Enertia Plus, Empulse 6.0 - 10.0 kwh, 2011 Zero to 2012 Zero) have been chemistry changes. Swapping in a different chemistry has the potential for larger gains than just swapping in an upgraded version of the same chemistry (lithium to nimh vs nimh to nimh). They may also be more complicated; may need a different battery management system or onboard charger; may need active cooling or venting or other airflow considerations; the new batteries may be packaged in a way that can't be made compatible with the old bike.
Still an option for the hobbyist to design a replacement pack. I'll be interested to see what people do with the original Enertias in 5-10 years when the battery packs are being replaced.