Back-patching const correctness is *very* expensive. Every 'const' you add 'over here' requires you to add four more 'over there'. The snowball effect is magnificent -- unless you have to pay for it. Long about the middle of the process, someone stumbles on a function that needs to be const but can't be const, and then they know why their system wasn't functioning correctly all along. This is the benefit of const correctness, but it should be installed from the beginning.
Short answer: CONST CORRECTNESS SHOULD NOT BE DONE RETROACTIVELY!!
