What you can or cannot skirt past using the law shouldn't be the issue.  People seem to want to go to great lengths to try and find some kind of loophole or consideration within the law to pass things off as their own creation.  Even as to go so far as to used advanced techniques - topology, approximation, or otherwise closely modeling around something to stubbornly just try to slip past the fact that unless you make something from scratch it is not your model and not your original work.

As has been noted, if you have the skill to do these advanced things and put a lot of emphasis  into being sneaky, why not instead focus that energy on making something that you can truly call your own?

When would a model become my own?
When you create it fully from scratch it is 100% your own model.  How can someone even argue otherwise?  If you did 90% the work, then 10% of it is not yours.

I have a model of a train paid for.
Often you pay for the right to use it for renders and animations, commercially or non-commercially.  I don't even think there is a problem with sticking bits on it or removing parts if you intend to use it in renders or animations.

The big issue comes with "do with it what I wish" - this is a pretty open ended statement.  Should you sell it?  NO.  Should you redistribute it?  NO.  Simply because there are parts of it that are still created by someone else.  Can you distribute some kind of add-on for it?  Most likely YES, as long as no part of the original model is redistributed.

Say you worked months on a great, intricate model that  you put a lot of love and care into.  You decide to sell it.  Someone takes your item and changes some parts of it, repackages it and then sells it and he's riding off of a percentage of work that you spend a great deal of time on.  He might even undercut you, further destroying your time and energy.  Maybe he decides to just give it away.  How would those scenarios make you feel?

The whole "this guy took the feet off of this model" or "that gal took the eyes of of this model" was just an easy way out and a blatant attempt to ride off of the work of others and it boggles my mind.  Why not emphasize getting better at your skill instead?  You know practice.  Learn.  Study.  But do it yourself and have the knowledge and pride  that you did it yourself and wondering about "how much you can get away with" never becomes an issue.

My 2 cents.

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