On the basis of an analytical mean-field model, we consider phase separation and coilglobule transitions in solutions of polymer chains with annealed excluded-volume interactions. In our model, the chain monomers are able to reversibly change their state from a hydrophilic to a hydrophobic one. This may mimic the behavior of polymers in solutions that contain amphiphilic molecules, e.g., globular proteins or surfactants, that are capable of cooperative association with the polymer chains. While the bare polymer chains are either hydrophobic or weakly hydrophilic, the formation of polymer/surfactant or polymer/protein complexes may strongly enhance the solubility of the polymers in water. We predict different phase behavior for solutions of originally hydrophilic or hydrophobic polymers mixed with amphiphiles. In the former case, the solution remains homogeneous at low concentrations of amphiphiles, whereas in the latter case it separates into a dilute and concentrated phase. The Theta -transition of the complexed polymers may be induced by variation either of the concentration of amphiphiles, of the temperature, or of the ionic strength of the solution. At low ionic strength the collapse of an individual, complexed chain with decreasing solvent strength or decreasing amphiphile concentration acquires the character of a first-order phase transition. At high concentrations of amphiphiles and high ionic strengths, we predict the possibility of the coexistence of two semidilute polymer phases, both for hydrophilic and for hydrophobic chains.