Change World Background in Time Very new to blender.

Can I change the world (background) and their different textures within one project?

I can create multiple worlds, but only use one of them in one project, there is no keyframing to this. Also, if I add multiple textures, they just get layered on top of each other rather than one texture being applied to one world.

To put it simple: I want to have a green blackground with cloud texture from frame 1 to 50 and then change into a blue blackground with noise texture from frame 50 to 100 and so on. This should be possible, right?

Hope I made myself clear.
Animate the World's material

You could create several world shaders and then mix them with a Mix Shader right before the output node. Mix Shader has an field named Factor that allow control of the percentage of each one coming out from the node and that can be animate.