MERLIN (Multi Element Radio Linked Interferometer Network) is a cornerstone of the UK's ground-based astronomical facilities, providing sub-0.1 arcsecond radio imaging, polarimetry, spectroscopy and astrometry. It consists of six telescopes distributed over central England with a maximum baseline length of 217 km. The sensitivity of MERLIN is currently limited by the narrow-band of the microwave links used to transmit data from the remote telescopes to the correlator situated at Jodrell Bank. At the heart of the MERLIN upgrade will be the replacement of these links with broad-band optical fibers transmitting data at up to 10 Gbps. In addition, new receivers in the 12 - 15 GHz band will be installed, the old low-frequency telescope at Defford will be replaced and a new broad-band correlator will be constructed. The result will be an array transformed, with a sensitivity of up to a factor of 30 greater than that of the current array, which will give the capability of new science at high frequency and high resolution.