Thanks for responding, Matthew. I finally figured it out. It was several materials settings. Apparently, in AC9 all of the materials pens were set to "element pen". Some in AC10 are set to a different pen, and I just happened to use several that were set to approx. the same line width. I didn't even realize that vectorial fills could be a different pen-a feature I will begin to use.
AC19(9001), 27" iMac i7, 12 gb ram, ATI Radeon HD 4850 512mb, OS 10.12.6