I have a number of apc rack PDU's, model 7862/7864's, which appear to only support only snmpv3 with DES/MD5 as the encryption/authentication protocol. The newer 8861 rpdu's support SHA/AES for snmpv3.
The AOS versions I show for the 7800 series pdu's are 3.x and 2.x, whereas the AOS version for the 8861 pdu's is a mix of 5.x and 6.x.
Is support for SHA/AES with snmpV3 something that can be added to the older hardware via a firmware update, or is that a restriction of the older hardware of thse older pdu's? I haven't found an answer yes or no on this from my searching.
Just another user here, but I may be able to help.
1) The units with aos versions 2 or 3 are an older generation of management card. 3.9.x is the latest firmware version for most products that use that management card. That card design is 10+ years old and the 3rd-party toolchain needed to build the firmware isn't a lot newer. APC went through a lot of effort to have the 3.9.x release work with modern (at the time) browsers, only to have Firefox break it less than 2 weeks after the firmware was released. Back when that firmware was released, APC still had a few models in production / under warranty that used that generation of card, so it made sense for them to do it. I don't know if it is even possible for APC to get a newer version of the toolchain at this point.
2) The units with aos versions 5 (blue UI) or 6 (green UI) are using the current generation of management card, which gets regular firmware updates. At this time, 6.5.x is the current version for most products that use that card.
3) You should probably update both types of units to the latest applicable firmware version. There are some security fixes in the updates as well as additional features / bugfixes. Note that some models have additional steps or cautions when updating from 5.x to 6.x firmware, which should be covered on the download page or in the documentation that comes with the firmware package.
In addition to what Terry explained, I wrote up some info on AOS/hardware versions in NMC1/NMC2 devices (AP7XXX series is NMC1, AP8XXX or AP7XXXB series is NMC2) yesterday for a slightly different question and it may be helpful for further understanding. https://forums.apc.com/spaces/6/racks-rack-accessories-cooling/forums/general/87240/aos-firmware-versions
In summary, unfortunately you won't be able to add SHA/AES to the older product and the firmware upgrades for the products are not interchangeable.
Choose a location