I have now made three versions of this mod.
The first version is in the post above and replaces the new human portraits with the old. This should be achievements compatible, don't know about MP as I've never played MP.
The next version re-adds the old human portraits as a new portrait. The stock human empires should use the new portraits but you can shoose the old ones if you like
steamcommunity.com
The last version does what
@Yurichenko was aiming at. The new portraits are available to choose but stock empires use the old ones
steamcommunity.com