  PSR J1846-0258 is an object which straddles the boundary between magnetars
and rotation powered pulsars. Though behaving for many years as a
rotation-powered pulsar, in 2006, it exhibited distinctly magnetar-like
behavior - emitting several short hard X-ray bursts, and a flux increase. Here
we report on 7 years of post-outburst timing observations of PSR J1846-0258
using the Rossi X-ray Timing Explorer and the Swift X-ray Telescope. We measure
the braking index over the post-magnetar outburst period to be $n=2.19\pm0.03$.
This represents a change of $\Delta n=-0.46\pm0.03$ or a 14.5$\;\sigma$
difference from the pre-outburst braking index of $n=2.65\pm0.01$, which itself
was measured over a span of 6.5 yr. So large and long-lived a change to a
pulsar braking index is unprecedented and poses a significant challenge to
models of pulsar spin-down.
