Fluorescent organic nanoparticles (FONs) based on aggregation-induced emission (AIE) are receiving increasing attention owing to their simple preparation, enhanced optical properties, and wide range of applications. Therefore, finding simple methods to tune the FON’s structural and emissive properties is highly desirable. In this context, we discuss the preparation of highly emissive, amorphous AIE spherical nanoparticles based on a structurally-simple molecular rotor and their sonochemical transformation into rhomboidal nanocrystals. Interestingly, the ultrasound-induced modification of the morphology is accompanied by a remarkable enhancement in the stability and emission of the resulting nanocrystals. A detailed characterization of both spherical and rhomboidal nanoparticles was carried out by means of several microscopic, crystallographic, and spectroscopic techniques. As a complement, the photophysical processes related to the nanocrystal emission were studied and corroborated via quantum mechanical calculations. In a nutshell, this work provides a unique example of the ultrasound-induced switching of morphology, stability, and emission in FONs