If you do such a thing, Cubase would normally tell you that there are other audio events in your project that will be affected and ask you if you wish to apply your changes to all or make a new copy to just alter the one event you’re working on.
If it doesn’t show you this dialog, you probably ticked the ‘don’t ask this again’ box at some point 
. I suspect you can undo this in the Cubase preferences somewhere. I don’t know where to look for it though, so if you can’t find it try the manual.