No easy fix if Khoros don't sort it.

I dunno if you could write a post, preview it, delete your present sig then paste in your "new one"?

You could have your "new" sig saved in an easy-to-find place ready to copy & paste each time you post. A right rigmarole I know but can't think of much else except.......

If your present sig is saved as an image somewhere and the link on your hosting site might be something like xxxxxx.img.1234.etc make your new one and save it. Note the file name you gave your original and then delete it altogether. Then copy and paste your new sig again and give it the same file name as your original sig image.

However, I think Khoros copy your sig etc and give it a completely new link so I dunno if the above would work.