  Supermassive black holes launch highly relativistic jets with velocities
reaching Lorentz factors as high as $\Gamma>50$. How the jets accelerate to
such high velocities and where along the jet do they reach terminal velocity
are open questions that are tightly linked to their structure, launching and
dissipation mechanisms. Changes in the beaming factor along the jets could
potentially reveal jet acceleration, deceleration, or bending. We aim to (1)
quantify the relativistic effects in multiple radio frequencies and (2) study
possible jet velocity--viewing angle variations at parsec scales. We used the
state-of-the-art code Magnetron to model light curves from the University of
Michigan Radio Observatory and the Mets\"{a}hovi Radio Observatory's monitoring
programs in five frequencies covering about 25 years of observations in the
4.8-37~GHz range for 61 sources. We supplement our data set with high-frequency
radio observations in the 100-340~GHz range from ALMA, CARMA, and SMA. For each
frequency we estimate the Doppler factor which we use to quantify possible
changes in the relativistic effects along the jets. The majority of our sources
do not show any statistically significant difference in their Doppler factor
across frequencies. This is consistent with constant velocity in a conical jet,
as expected at parsec scales. However, our analysis reveals 17 sources where
relativistic beaming changes as a function of frequency. In the majority of
cases the Doppler factor increases towards lower frequencies. Only 1253-053
shows the opposite behavior. By exploring their jet properties we find that the
jet of 0420-014 is likely bent across the 4.8-340~GHz range. For 0212+735 the
jet is likely parabolic, and still accelerating in the 4.8-37~GHz range. We
discuss possible interpretations for the trends found in the remaining sources.
