Testing with C8.5 pro 64bit, Windows 10 2015.
After having saved a custom preset in a virtual instrument I noticed that I did put in some wrong attributes. I loaded the preset again and decided to do another save. In the saving dialog I corrected the attributes. When saving Cubase prompted to overwrite the existing preset - did that. Attributes still wrong.
As always there is a workaround in Cubase - to change attributes by starting up media bay and change the attributes in the properties pane, but as always these workarounds will take you right of out your workflow/focus again.