I have an assembly imported in Visualize using the automatic grouping. Let's say I have a few components using the same apperance. If I wan't to change that apperance I could, in Visualize 2017, copy the the desired apperance and then right click on the apperance I want to be changed and select "assign apperance. ..". It worked like a charm!
When trying the same thing i Visualize 2018 SP2.0ev it doesn't work. I can go to the model tab and select the components there and assign the apperance but it doesn't work from the apperance tab.
This was really handy, beeing able to say that everything that was yellow low gloss plastic in SolidWorks should be something else i Visualize.
Until then (as a workaround), you need to make a selection in the viewport (using either Part or Appearance Selection Mode in Selection Tool, and choose "Edit > Assign Appearance" to assign an appearance that was previously copied to the clipboard!