To all those questioning what you can and cannot do to the layout, I would suggest not worrying about changes that are within a 10-20% threshold. For example, if you need to move a doorway 16-32 units and then widen a wall by 32 units to make things line up properly with your changes, don't worry about it. Remember, it is IMPOSSIBLE to detail a map, especially when making the architecture nice, not to change things around like this. Think not in terms of keeping the dimensions the same everywhere but keeping the gameplay the same. For example, if a sentry location is moved back 64 units but it can still see just as far (if it's looking at a wall for example) that is not a layout change as it does not affect gameplay.
It is rare and specialized situations affecting only the most skilled players of only a few classes when 8 - 32 units difference changes a tactic. (demoman quickly popping around corners to throw stickies for example). Heck, even the overhangs in CP C and A-building can be mostly removed without affecting gameplay.
Still, I am curious, what if we added something like an overhang with support beams coming down off it that divides a path into two without truly adding new areas? Would this be a layout change? I would think not. Would adding a few soldier/demoman perches be a layout change? I would think not. Would changing the slope of the highly-graded clip brush be a layout change? I would think not. The majority of the changes people are asking about would not change the flow or balance of the map as a whole and would only make minor changes to individual firefights in some specialized circumstances.
The meaning of "layout" is not "shape of areas and routes" but rather "type and character of situations produced by the areas and routes."
Think back on your own maps and the way they evolved through alpha, beta, etc. You might, in alpha, have a building rooftop where an engy can build a sentry, and then in beta you might add a duct on top of it which would block/change exactly were sentries can be built, but it does not change that sentries can be built there.
Everytime you make a change you question, compile and run around the level and see if it FEELS different, if there is a major change in what you feel capable of DOING as each class, then that is a layout change.
You don't JUST want to texture and add models and sounds, the first step in detailing a level is ARCHITECTURE, and you can't do that without making some changes. Look at that massive wall dividing the starting area from the A-B route for example: if you just texture that, you have not detailed it.
If the shell of a level remains bland then all the textures and props in the world will not save you.