I wonder: Is there AN EASY WAY to take a whole scene and copy/create it with similar content in a different projection (to be able to properly publish on ArcGIS Online)?
One way would be to export all your shapes to a .gdb file. File -> Export Models -> FileGDB -> Export Features = Shapes only. Then, you can import this into your other scene which has a different coordinate system. File -> Import. If you set Export Features to include the models as well, then you'll get the models too. With shapes, you can always regenerate the rules. I suggested to export shapes in case you want to assign different rules to the shapes.
thats a path to follow ... something like "change projection" would be much nicer :-)
Actually, now that you mention it, you could just change the coordinate system. Edit -> Preferences -> Scene -> Scene coordinate system.
But, if you want to put the contents of your scene into another existing scene that may already have stuff in it, then you'll want to do the export/import approach.
Retrieving data ...