To stay compatible with my various clients, I have all 64-bit releases of SolidWorks (2014 to 2018) installed on my PC.
That means I also now have 3 releases of Visualize installed (2016 to 2018). As a result I now have 3 different Visualize Content folders.
Because Visualize does not change file formats with each release, I don't think there is any need to keep the older Visualize Content folders?
I can just copy older projects into the 2018 folder, plus any unique appearances?
Then can I uninstall Visualize 2016 & 2017?
I know there is a 2018 feature to better link to SW but only for SW2018. Not a problem for me because if I was rendering for a client with an older SW release, I would just copy his part/assembly files to a new 2018 CAD folder.
Custom content you might have saved/exported from 2018 will not be compatible with 2017. e.g. a custom appearance exported from 2018 won't work in 2017.
But the native SOLIDWORKS Appearances that ship with 2018, and that are on the cloud, are all created in 2017 so you'll still be able to use them without any problem.
As for downgrading your license, I don't think it should be needed (if even possible...I'm not sure). If you save a project using "professional-only" features, the project will still open in Standard. Said features will either not be present, or un-editable, but otherwise shouldn't cause any problems. Using the matrix to guide as to what features to use/save in the project is still probably a good idea.