In the present paper, we study the limit of zero mass in non-Abelian gauge theories both with Higgs mechanism and in the nonlinear realization of the gauge group (Stückelberg mass). We argue that in the first case the longitudinal modes undergo a metamorphosis process to the Goldstone scalar modes, while in the second, we guess, a decoupling process associated to a phase transformation. The two scenarios yield strikingly different behaviors at high energy, mainly ascribed to the presence of a massless Higgs doublet among the physical modes in the case of Higgs mechanism (i.e. not only the Higgs boson). The aim of this work is to show that the problem of unitarity at high energy in non-Abelian gauge theory with no Higgs boson can open new perspectives in quantum field theory. 1 After the work of J.M. Cornwall, D.N. Levin, G. Tiktopoulos, M.S. Chanowitz and M.K. Gaillard, the relation between the S-matrix elements for longitudinal modes of the gauge fields and those of the Goldstone bosons has developed to a somewhat more complex result than the one implied by the theorem on the point transformations of fields in scattering theory [17]. The present work adds more consequences to the discovery of the above mentioned physicists. Thus, we choose to denote the theorem by their names.