Since hard subtitles are located in a specific block, is it not possible to black out (delete) the block and then add different subtitles in the same location?
If what you want is to show another, soft subtitle, AFAIR you can. If it’s only to see in a computer player, you can set the soft subs background to black. This will cover most of the burned-in subs, depending on timing and line length. For example, in smplayer you can set opacity to max to achieve this.
If you want to burn new subs you have to use ssa subs and adjust the background color in the .ssa file.
I think I saved an example, if I find it I’ll post here.
Thank you. I will look into smplayer opacity.