Several places now I have discovered odd changes to the cycles materials I had stored with the initial launch version. One for sure: on the Math node the multiply has changed to ceiling. I've also noticed (I am pretty sure) that my image maps have changed from flat projection to cube. I think, with less certainty, that relationships between the nodes were lost, too

Not happy that my stored materials are not backwards compatible. I noted that you've adopted Agile software practices. You must have also adopted 100% automated testing to support this. Please add backwards compatibility regression tests for the cycle nodes storage.