It's worth noting that I basically used the original manual for Astrosmash! as a design doc.
The only really major gameplay change I made was to bump the 1-Up threshold to 2,000 up from every 1,000 points. And that was only after finding it just a bit too easy to rack up 40-50 lives as you push past 100,000 points.
Oh, and the UFO also has a big change, too -- it fires the usual energy bursts at you, but once in a while it will slow down and charge it's big laser! Keep moving to avoid being locked onto! I was going to remove this to keep the game pure to it's roots, but damn if I don't love how that big beam turned out. I couldn't just throw it out! ;)
Other changes mainly involved adapting the game to a 16:9 playing field, from the original's 4:3, and changing the abstract 'spinners' into the more tangible idea of atomic 'bombs'.