Go to the /common folder and open the file defines.lua in a good text editor such as Notepad++ (or plain old Excel if you must). Go to line 895 (or nearby depending on patch version) and edit the value for EARLY_PORTRAIT_AND_UNIT_BEFORE_YEAR from the current 950 to any time after the game ends, say 1500. Save file and there you go, early portraits will never go away.
The usual caveats apply - this will break Ironman, make a backup of the defines.lua file, best done as a personal micromod etc.