Set Image(Texture) Dynamically On Specific Part Of Sphere Using C#

Hello Everyone,

I am using Unity 3 Version 3.3.0f4.
I am very new to Unity3D and also 3D concept. But I am good at programming with C#(.net).
I am creating application same as Google Earth in Unity3D with C# scripting.

I have taken one Sphere and give a simple World Map Image as texture to it.
I have aslo written script in C# for Rotation and Zooming the Sphere like Google Earth.

Initially upto some Zoom-In level I want to keep the same Texture. When Sphere will Zoom-In after some level at that time I want to change the texture which can dispaly more detail veiw of earth location. This things will be repeted upto last Zoom-In level.

I am getting Longitude and Latitude value of Sphere on Mouse Over and Double Click event.

For that I want to set Image of specific part of World Map on the Some Specific Portion of Sphere only.
Like wise using C# scripting I want to change images as per users interations with Sphere.

Can anybody suggest me how can I accomplish this thing, Since long time I am trying to do this thing.
If anybody can give sample code then it will be better.

Thanks in Advance.

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