It really depends. This was an over paint from many moons ago:
In the real world, NJDot just overpainted all the former BN cars that came to the shore via the CNJ in the early 70's. I remember clearing seeing the swoosh from the BN under the dark blue paint that was the standard DOT color until it was rebranded NJT. PC and later PRR overpainted their locomotives as evidenced by old paint schemes peaking through as the paint started fading off the upper surfaces.
For models, I do agree that painting over existing paint does need some prep work and removing the paint is the best practice. I just don't always do that myself.
