The Epiphone SG G-400 is one of the best guitars to make later upgrades. I have a Epiphone SG G-400 that I made myself a lot of upgrades and it is right now my number one guitar. It sounds much better than most of the guitars on the market. Of course that the upgrades also cost a lot, but it worth every cent.
The guitar is almost totally upgraded I will list the upgrades from headstock to the body:
-Gibson Deluxe tuners (Kluson style this tuners are better made than Grovers, the Grovers were alright but the Gibsons improve the tone)
-Graphitech TUSQ Nut (It's amzing how it changed the sound, much clear, defined, brighther and open)
-Gibson 490R pickup (excelent pickup no need for further explanetion)
-Gibson 498T pickup (beyond excelent)
-Gotoh bridge (very good bridge the original tunomatic Epiphone bridge is ok, but the Gotoh improved the stability and intonation)
-CTS 500K pots (changing the pots improved the tone incredibily, make it more open, clear and rounder bass)
-Sprague Orange Drop 0.22 440V capacitors (for a better tone control it makes the tone knob more usable, with all this upgrades the sound became very bright so you have to control it)
With this upgrades my guitar's tone is head to head with the top guitars on Earth and I realy mean it once I compare it with a top custom 73' Gibson.
And still is a Epiphone SG G-400, the tone is amazing, but nobody wants to steal it.