Update: I got home started the daw, expecting to spend some time trying to diagnose a 64 bit despondent session drummer and gosh it just worked. So that's cool.
 
I had intended then to make presets for each session drummer instance in each mixed song to take over to the 64 bit side but my trusty 32 bit system now won't load any projects without the dreaded reset button crash so I gave up again as I've had enough of computer puzzles for one day.
 
Maybe tomorrow.

I'm calling this "Solved" although I'm not pleased with having to migrate setting from 32 to 64 for SD3 when TTS-1, AD Keys, Kontact, sfz player! even all just work. I don't know what makes SD so special.

I don't either. If the original project used Session Drummer 2 it would be a different matter but the move from 32bit to 64bit for the same plug-in should be seamless.

It's why for years I have dreaded this conversion though it's not as bad as I thought it might be.
The news that momentum was 64 bit only and 98% of yous are 64 bit made me fear as I did running Win 7
that my days were numbered. Now if I could just get back to the way it all was before.