Now, GPUgc only updates visible levels. When a level that was not rendered last frame is now visible it partially or fully update it.
GPU Geometry Clipmaps Demo 25.03.2007
GPU Geometry Clipmaps Source 25.03.2007
Edit : Updating visible levels only boost FPS during motion at high height. On a low height, there is less update calls but they have more work to do (GPU (synthetize)/ CPU (decompress)), the user might feel some freeze during motion on downmarket CPU/GPU. Updating all levels prevent freezes but it's slower when user is at high height. ( choice is yours ).
Note : if you want to try using normal maps size twice clipmap ones change
"TexNormalSize = TexClipSize * 1;" to "TexNormalSize = TexClipSize * 2;"
In GPUGeometryClipmap(). In ComputeNormals.fx change
"p_uv2 = floor(p_uv2 + 0.5);" to "p_uv2 = floor(p_uv2 + 0.25);".