Fused particle fabrication (FPF), an extrusion-based additive manufacturing (EBAM), is recently introduced as a cutting-edge upcycling technology that provides a fresh avenue for sustainable waste management of semicrystalline thermoplastics and diversifies the applications of recycled thermoplastics. Despite having tremendous potential, FPF application to semicrystalline thermoplastics encounters substantial hurdles rooted in the intricate microstructural transformations during the processing. The dynamic transformations in microstructure lead to warpage through differential shrinkage and consequential decline in the mechanical and thermal stability of the final product. Acknowledging the pivotal role of microstructure in determining the abovementioned properties, the present study investigates on the microstructural evolution and properties change of a semicrystalline thermoplastic, i.e. poly(ethylene terephthalate (PET), following multiple (four) cycles of FPF to establish the processing-microstructure-properties relationship. The study discerns alterations in nucleation, crystal growth, crystal dimensionality, crystallite size, overall crystallinity and microstructural contents (rigid and mobile amorphous components). The findings demonstrate that repeated FPF cycles significantly affect PET microstructure and properties. Initial FPF cycles promoted nucleation, while further cycles enhanced crystal growth with reduced crystallisation rates in the PET microstructure. The microstructural alteration has been identified as an underlying factor and significant contributor to the increased density and volumetric shrinkage, diminished tensile properties and improved thermal stability in the final part following each processing cycle. The altered microstructure decreases specific volume and increases volumetric shrinkage, suggesting a potential warpage of PET-printed parts after the 4th FPF cycle. A reduction in tensile strength, ductility, and brittleness was observed after multiple cycles, alongside an increased thermal-oxidative stability of the recycled PET. The research findings delving into the microstructural insights of semicrystalline thermoplastics will assist in yielding printed parts endowed with tailored mechanical and thermal properties with minimum warpage. The resultant knowledge will facilitate the development of the FPF technology as a sustainable upcycling tool for semicrystalline thermoplastics, marking a stride towards optimised EBAM practices.