* Intro: picture of man ("No, go to star") is flipped horizontally versus ver.b * "game over" image has different palette * Intro: spinning "Space Bomber" at bottom right is silver instead of red. * Lives counter are static images and not animated (ver.b presumably reuses a zoomed player sprite with move left/right animation frames) * It's missing the intro between worlds and on all clear ("You stinking aliens betrayed us!", clearly developed by rehashing images from the intro in ver.b to add an element of story) * It's missing the 'rankings' on all clear ("CAT-2" etc.)
Gameplay (based on a random level sampling):
* The third wave of bullets from boss 2-4 are bigger than in ver.b. Large circles instead of the classic-Psikyo ovals * Bullets from flying saucers on 1-4 are bigger than ver.b also * More cases like this
I'm sure the rest is bug-fixes and gameplay tweaks. Unlike Lode Runner - The Dig Fight it's not a complete overhaul though.