We consider a problem of relativistic jets loaded by charged pairs (electrons and positrons) due to two-photon conversion. The most important effect of the charged pairs loading is screening of electric and magnetic fields in magnetohydrodynamic outflow. We show that pairs created with center-of-mass velocity less than the drift velocity of a bulk motion disturbs locally (in the domain of pair creation) electric and magnetic fields. This disturbance is calculated self-consistently, and it is shown that even input of about 1% in particles number density may change the bulk flow Lorenz factor up to a factor 2, the effect being more pronounced for the faster jets. At the same time, the flow magnetization grows, with the growing factor depending on the initial bulk Lorenz factor. The pairs created with center-of-mass velocity greater than the jet bulk velocity enhance locally the electric and magnetic fields such as to accelerate the flow, acceleration being more effective than the deceleration. The astrophysical application concerns the recently observed by the MOJAVE program relativistic jet deceleration and acceleration on scales up to 100 pc. The proposed mechanism of pair creation due to two-photon conversion of soft radiation field and hard photons of jet radiation may account for the observed rates of acceleration. The effect may also be important as a source of radiation sites: if the jet decelerates or accelerates locally, the interaction of the overcoming flow with the decelerated domain may lead to instabilities and radiation.