Tunability is one of the key aspects of any laser system. In High-Gain Harmonic Generation Free Electron Laser (HGHG FEL) the seed laser determines the output wavelength. Conventional scheme of tunable HGHG FEL requires tunable seed laser. The alternative scheme  is based on compression of the electron bunch with energytime correlation (chirped bunch) in the FEL dispersive section. The chirped energy modulation, induced by the seed laser with constant wavelength, is compressed as the whole bunch undergoes compression. In this paper we discuss experimental verification of the proposed approach at the DUV FEL [3,4] and compare experimental results with analytical estimates.