Terragen isn't going to be very helpful in creating the base maps, and that will really be the first thing you have to do. So use an image editor or other terrain editor (something with a lot of explicit control, like Leveller for example), or find a source of existing data that's good enough for your needs. You don't necessarily need one image for every frame of your eventual animation, but you do at least need enough to show the major movements and stages of development, similar to the somewhat jerky motion in the animation you linked to. Critically, you will need the data to be in a spherical/polar projection, ready to be mapped to a sphere.
Once you have at least a series of maps, you can then load those in Terragen as color and/or displacement to create your terrain. To morph between states you could potentially use a Merge Shader and adjust the merge amount over time (animate the Merge value). You would have to do this across multiple merge shaders and image map loaders for a full animation. Alternatively, you can take your maps and find a 3rd party application to create interpolation (in-between) frames and end up with a full series of images, 1 for each frame you plan to render, then load an image sequence in a Terragen Image Map Shader.
So basically, the biggest chunk of the work is probably going to take place in data gathering/generation, most likely outside Terragen. As long as you get the data into spherical projection, you should be able to apply it to the planet in Terragen. Once you have the data, we can talk about more specifics on how to apply it and blend between shapes, etc.
- Oshyan
