Similarity laws are often employed when characteristics of two or more discharge plasma systems are compared. The classical similarity laws were previously validated and applied for weakly ionized plasma discharges. However, the classical similarity laws will not be valid for all plasma regimes. Especially for strongly ionized regimes, scaling laws are not well elaborated. In this study, transition characteristics of low-temperature plasma similarity laws are evaluated from low to high ionization degree regimes. The similarity relations of plasma density and ionization degree are obtained for geometrically similar gaps. The deviations from classical similarity laws are observed, which are gradually enlarged as the ionization increases from low to high degrees. The transition characteristics are affected by the significance of nonlinear reaction processes (such as three-body collisions), resulting in that the classical similarity relations hold up to a higher ionization degree at low pressure than that at high pressure. The time-dependent scaling characteristics of species densities and electron kinetic behaviors in geometrically similar gaps are also examined. The results are beneficial for utilizing the similarity laws in a wide range of ionization degree regimes, which is essential for correlating plasma characteristics in geometrical similar vessels of various scales.