What happens when I takeoff or or land at KLAX (the latest current version) with the HD Textues option set to 2048? Will the KLAX textures just drop all the way down 1024?
No, they'll work at 2048x2048, because they includes all Mipmaps levels from 4096x4096 down to 4x4, so FSX will pick up the correct one based on your viewing distance, but not exceeding the "Texture Max Load" value, which is what the Addon Manager switch controls.
However, if you never had problems with memory at KLAX, you might choose to leave the Addon Manager setting at 4096, so KLAX will work at 4096, while CYVR will still work at the resolution you choose in its installer, because its textures has been *built* by the installer at the specific resolution you selected, so they won't be displayed any higher than that.
Some examples:
You selected 2048 for Ground and 1024 for Shadows while installing CYVR:
With the Addon Manager set to 4096, KLAX will display at 4096, while CYVR will display Ground at 2048 and Shadows at 1024
With the Addon Manager set to 2048, KLAX will display at 2048, while CYVR will display Ground at 2048 and Shadows at 1024
With the Addon Manager set to 1024, everything will display at 1024
You selected 4096 for Ground and 2048 for Shadows while installing CYVR:
With the Addon Manager set to 4096, KLAX will display at 4096, while CYVR will display Ground at 4096 and Shadows at 2048
With the Addon Manager set to 2048, everything will display at 2048
With the Addon Manager set to 1024, everything will display at 1024
In any case, since the Addon Manager allows to control the texture resolution without having to restart FSX, nothing prevents you to set it to 4096 on an airport, and 2048 or 1024 in another one. That also depends which airplane you flying.