So I have a K2i but wanted to replace the main font with the latest-and-greatest 5.40 generation from the k3+ updates.
Steps: Installed font hack.
Copied in fonts using these naming conventions.
(old) Helvetica* -> Sans*
(new) Caecilia_LT (not Cond) -> Serif*
(old) Kindle_Blackbox* -> Mono*
Restart. Success, I'm now on the latest version. Not that I can tell any real difference.