After the discovery of more than 100 exosolar Jupiter-class planets the detection of Uranus-size and large terrestrial-like bodies will be the next major step in the exoplanet field. Space-borne telescopes like COROT and Kepler using high precision photometry and the transit technique, will have the capability to detect exoplanets with sizes of 1 - 4 Earth radii at distances between 0.3-1 AU. Current theoretical models indicate that the discovered large exoplanets orbiting close to their central star may have either migrated inward from greater distances, or may have formed at their present orbit. After the observation of an extended upper atmosphere and large atmospheric hydrogen loss rates of HD209458b indicating that this planet is under hydrodynamic conditions model simulations of atmospheric stability for hypothetical smaller exoplanets with the size of Uranus, Neptune or even terrestrial bodies are essential. We show how thermal and various non-thermal atmospheric escape processes may effect the evolution of planetary atmospheres and even their habitability. Since all escape processes depend on the orbital parameters, the evolution of the radiation and particle environment of the host stars, stellar data are included in the simulations. Interestingly it is found that migrating Uranus-class exoplanets may lose their entire dense hydrogen atmospheres by these loss processes and can evolve into a new type of terrestrial planet, after the development of secondary atmospheres by out-gassing their remaining ice-rocky cores.