Two new PAH processes are proposed, which can cause dehydrogenation and "rounding" of PAH molecules. The reaction pathways for both the processes involve decyclisation of a 6-member ring present on the PAH surface. 1,6-hydrogen migration in the bay region formed by nearby carbon atoms provides an alternate route for both the processes to proceed. The energetics and kinetics of the proposed processes are investigated using density functional theory and transition state theory, respectively. The B3LYP functional with the 6-311++G(d,p) basis set is employed for the geometry optimisation and vibrational frequency analysis of all the chemical species and the transition states. The current PAH growth mechanism is extended by including the new PAH processes. A detailed PAH growth model, the kinetic Monte Carlo-aromatic site (KMC-ARS) model [Raj et al., Combust. Flame, 156:896-913, 2009] is used to study the growth of PAHs with the extended mechanism. Computed ensembles are generated for large PAHs present in a C2H2 flame with 70-320 carbon atoms, and are compared to the experimentally observed ensembles. The inclusion of the new PAH processes in the chemical mechanism is found to improve the predicted composition of the large PAH molecules, especially for the PAHs with 70-200 carbon atoms.