how much/what exactly would I have to change before copyright doesn't become an issue?
As much as it would take for you to think, "Ah I have created something totally original here!"...?
Really, this is just looking for the loophole again: if you make the characters, settings and what-have you easily identifiable as something else, you will always be open to possible action by the other property's rights holder, and with all the risks that that entails (a court may find in the matter for either side, but why risk it?). Just changing things a little to try and get round the problem isn't really going to do it, and certainly isn't the protection that doing your own original thing from the ground up would be.
to create another entirely different image of it isn't taking anything physical away from the owner
Well it takes away their control, which is very important, and a violation of their rights. Just because the thing taken isn't tangible doesn't make it valueless, or unimportant.
It could be argued that it is more important in some respects: you lose a car, you can get another one. Someone does something with your idea without permission, and nothing can return it to you intact, or take away the existence of what someone else has done.
As an example, say you appropriate the likeness of my hugely popular (yet entirely hypothetical) Cushion-cover Guy
, the crime-fighting nemesis of Sofa City; you do this as an hommage, with the purest of motives, but it looks like my work. In doing this, you draw him battling Lamp Lad, a character I didn't create.
What you don't know is that I am at work on my magnum opus, C-C G vs The Deadly Lamp-Shade
What am I to do? If I bring out my
book, it will look like I
am copying you
... You have effectively damaged my ability as artist and creator to do things the way that I want, without recourse to a re-set switch to return things to the way they were.
No matter what I do to explain, someone somewhere will say I stole from you, or that my ideas weren't original, or will believe that I wouldn't have come up with Deadly Lamp-Shade if it hadn't been for you; an intangible crime then leads to other problems, with my loss of reputation, sales, creative freedom, and what not. The genii doesn't go back into the bottle...