We have exploited angle-resolved photoelectron spectroscopy to study changes in the quasiparticle excitations of CuO2 planes as a function of the carrier density, in optimally doped and underdoped Bi2Sr2CaCuO8+x and in Sr2CuO2Cl2. We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Fermi surface away from optimal doping. We have also found that the carrier density has a nontrivial effect on the symmetry properties of these states.