First create an image based on your planet texture map that can be used to mask areas on the planet. It would sort of look like an alpha channel of the planet's continents. The continents would be white and the ocean area black. Load the image into an Image Map shader and set its projection to spherical, then change the Y axis Position value to match your planet's Y axis Centre value.
Connect the Image Map shader to the Mask Shader input on one of Terragen's procedural noise nodes, such as the Power Fractal shader v3. Now you've constrained the surface displacement to just the continents.
To create different terrains, just change the noise node's Seed parameter. Of course, you can change any of the noise node's parameters to further change the look of the terrain.
If you want to shift the positions of the continents, try using a Fractal Warp shader to distort the Image Map used to mask the continents.
