Well let's assume you are correct. Just go download the publicly available android update and put it on your phone. If it's just a software component like you say, then it should just work.
What's more so : there is no publicly "available android update" that includes a patch for 4.3-.
If it were in existence, rebuilding the components is easy, getting them on a "unlocked -as in bootloader- phone" is the challenge. But if the bootloader is unlocked, chances are the user is tech savvy and the device is in "the supported devices list" of some custom android project out there (CM/AOKP/OMNIUM/PA to name few).
Those who can't write, write manuals.