We present results from the archival Chandra observations of the 0.3 s X-ray pulsar PSR J1846-0258 associated with the supernova remnant (SNR) Kes 75. The pulsar has the highest spin-down luminosity (Edot = 8.3e36 erg/s) among all the high magnetic field pulsars (HBPs) and has been classified as a Crab-like pulsar despite its magnetic field (5e13 G) being above the quantum critical field. It is the only HBP described by a non-thermal Crab-like spectrum, powering a bright pulsar wind nebula (PWN). Our spectroscopic study shows evidence of spectral softening (photon index = 1.32 to 1.97) and temporal brightening (unabsorbed flux = 4.3e-12 to 2.7e-11 erg/cm^2/s) of the pulsar by ~6 times from 2000 to 2006. The 0.5-10 keV luminosity of the pulsar at the revised distance of 6 kpc has also increased from 1.85e34 to 1.16e35 erg/s, and the X-ray efficiency increased from 0.2% to 1.4%. The observed X-ray brightening and softening of the pulsar suggests for the first time that this HBP is revealing itself as a magnetar.