This is an interesting example with what can happen if a ruler is replaced: in 1925 Sultan Ahmed was deposed, and pending the issue of new stamps the new ruler of Persia, Riza Khan Pahlevi, ordered that the portrait of the deposed Shah should be obliterated on all stamps by any convenient method. Lewis (in his book as cited in an earlier post) writes:
"The degree of obliteration rather naturally varied with the sympathies of, or means available to, the person doing the obliterating. Thus we can find square or circular cork (?) impressions, thumb or finger prints, pen or pencil scratches or doodles, and so on; either thinly or thickly applied."
An association I had when seeing these stamps was with a French stamp I have of Maréchal Pétain, after the 2nd world war his image was not to be seen any more, a liberated country wanted to de-face the leader of the oppressors. Well, no shortage of corks in France!
My guess is there are many more examples of rulers on stamps that have been obliterated in a heavy fashion when the times - and their luck - were changing. I will be on the look out for them. If you know of any, let me know! I may add them to this post.