  One of the tenets of the radio pulsar observational picture is that the
integrated pulse profiles are constant with time. This assumption underpins
much of the fantastic science made possible via pulsar timing. Over the past
few years, however, this assumption has come under question with a number of
pulsars showing pulse shape changes on a range of timescales. Here, we show the
dramatic appearance of a bright component in the pulse profile of PSR
J0738-4042 (B0736-40). The component arises on the leading edge of the profile.
It was not present in 2004 but strongly present in 2006 and all observations
thereafter. A subsequent search through the literature shows the additional
component varies in flux density over timescales of decades. We show that the
polarization properties of the transient component are consistent with the
picture of competing orthogonal polarization modes. Faced with the general
problem of identifying and characterising average profile changes, we outline
and apply a statistical technique based on a Hidden Markov Model. The value of
this technique is established through simulations, and is shown to work
successfully in the case of low signal-to-noise profiles.
