The average pigment function is all we need to do this. Lets declare some textures to play with:

#declare basetexture = texture {
   pigment {radial frequency 8} 
   finish{specular 1}}
   
#declare cleartexture = texture { pigment { rgbt 1 } }

#declare secondtexture = texture { 
   pigment { granite } 
   normal { granite } }

Image one shows a simple sphere with basetexture applied, Image two shows the same simple sphere with secondtexture applied to it. I didn't bother rendering anything with cleartexture applied to it, because the image is quite boring.

Now we just need to create the texture. The example will give you an animation, of course. You want to plug in a number between 0 and 1 for texture_clock. When texture_clock is 0, the first pigment in the list will be applied, and when texture_clock is 1, only the second pigment will be applied. It's when texture_clock is between the two that something interesting happens.

#declare texture_clock = clock; // 0 to 1

sphere { 0.0, 1 
         texture { average 
                   texture_map { [(1- texture_clock) basetexture ]
                                 [(texture_clock) secondtexture ] } } }