Is it possible for a firmware upgrade to a camera (a Leica M9) to enable it to accept SDXC memory cards above the 32GB limit of SDHC, or is this change from SDHC to SDXC actually something which would require a change of hardware?
Should be firmware because in terms of hardware they are physically the same. That of course doesn't mean that a company will ever actually release the needed update.
As an aside, I teach photography and am a professional artist and would suggest that for almost all uses, SDXC cards aren't a good idea (time-lapse being the possible exception). The problem with large cards is if they fail you lose everything. If you stick with 8gb cards and change every hour or two then at worst a corrupt card means you haven't lost the whole days work.