My understanding of SDXC is that unlike the SD/SDHC hit, the current version of the SDXC cards (Using SD3.0 format) should work in SDHC readers if they can talk that format (may require a flash update). There is some doubt as to the capability of them to read the (as yet unimplemented) SD4.0 format though, even with firmware updates.

One thing to note is that technically the capacity for SD (SD1.0) was 512M, but using some tricks (changing sector sizes mainly) you can boost that up to 4G (using the SD1.01 format). The same trick can expand SDHC up to 2T, so SDHC is not necessarily capped at 32G. SDXC is capped at 2T, and the SD3.0 definitions show the most expansion using the current tricks would be 16T. The SD4.0 format for SDXC may expand it up that high, but that's the current limit for that format.