Eric Stevens
As nospam has so often told us, Lightroom (and other software using side car files) do not actually change the file being edited until it is in the process of being exported. In most case, all you see on the screen is a simplified simulacrum of what the edited file will look like, when the editing instructions are executed.

Once you export the file - that's it. You cannot reverse the changes. All you can do is edit the original all over again but this time slightly differently.

nospam
which means the changes are reversible.

Eric Stevens
You are not reversing the changes: you are substituting for them. Surely even you can see that?

Now it's interesting that Lightroom does incorporate something a little bit like the reversible process that Floyd was talking about but neither nospam or Savageduck seem to realise the fact. See http://tinyurl.com/p5sus42 From blur to sharpness on the one slider. But this is not actually a reversible process: it's a change in the instruction to the final edit which will only be executed when the image is exported.

nospam
not only do i realize it but that's what i've been saying all along.

you are *so* confused.

Eric Stevens
And I have pointed out that you cannot reverse a change which has not actually been made. Even if it is reversible, you can't reverse something before you have done it.