Context. The role of magnetic fields in gamma-ray burst (GRB) flows remains debated. If of sufficient strength, they can leave their signature on the initial phases of the afterglow by substantially changing the backreaction of the flow as a consequence of its interac- tion with the external medium. Aims. We attempt to understand quantitatively the dynamical effect and observational signatures of GRB ejecta magnetization on the onset of the afterglow. Methods. We perform ultrahigh-resolution, one-dimensional, relativistic MHD simulations of the interaction between a radially ex- panding, magnetized ejecta with the interstellar medium. We require ultrahigh numerical resolution because of the extreme jump conditions in the region of interaction between the ejecta and the circumburst medium. We study the complete evolution of an ultra- relativistic shell to the self-similar asymptotic phase. Results. Our simulations demonstrate that the complete evolution can be characterized in terms of two parameters, the ξ parameter introduced by Sari and Piran and the magnetization σ0. We use this fact in producing numerical models in which the shell Lorentz factor γ0 is between 10 and 20 and rescaling the results to arbitrarily large values of γ0. We find that the reverse shock is typically weak or absent for ejecta characterized by σ0 >∼ 1. The onset of the forward shock emission is strongly dependent on the magneti- zation. On the other hand, the magnetic energy of the shell is transferred into the external medium on a short timescale (of several times the duration of the burst). The later forward shock emission contains no information about the initial magnetization of the flow. The asymptotic evolution of strongly magnetized shells, after experiencing significant deceleration, resembles that of hydrodynamic shells, i.e. they enter fully into the Blandford-McKee self-similar regime.