I removed all the upgrade solderings, resoldered the single pre-upgrade link, replugged the Rom original chips and et voila - functional plain H3000 again!
http://www.southseascave.com/wp-cont...00_is_back.jpg
One thing is different, though. The machine only has programs 105, 106, 107, 109, 110 and 253.
I powered the H3000 off and replaced its original second Rom chip which the upgrade set's. Will I get a kajillion programs now? Restart, reset... nope, still only 105, 106, 107, 109, 110 and 253. These presets must be on the first, OS Rom. And yes, when I powered up with just the OS Rom, these 6 presets were still available. So the second chip isn't "coming in". Powering up with just the upgrade OS Rom produced no love, so I put the old OS Rom back.
(To think all this heap of trouble is because Eventide designed the Rom area of a $3000 unit in a way to require soldering just to accommodate more Rom chips...)