  Since their first introduction, Quantum Reference Frame (QRF) transformations
have been extensively discussed, generalising the covariance of physical laws
to the quantum domain. Despite important progress, a formulation of QRF
transformations for Lorentz symmetry is still lacking. The present work aims to
fill this gap. We first introduce a reformulation of relativistic quantum
mechanics independent of any notion of preferred temporal slicing. Based on
this, we define transformations that switch between the perspectives of
different relativistic QRFs. We introduce a notion of ''quantum Lorentz
transformations'' and ''superposition of Lorentz boosts'', acting on the
external degrees of freedom of a quantum particle. We analyse two effects,
superposition of time dilations and superposition of length contractions, that
arise only if the reference frames exhibit both relativistic and
quantum-mechanical features. Finally, we discuss how the effects could be
observed by measuring the wave-packet extensions from relativistic QRFs.
