The theory of group classification of differential equations is analyzed,
substantially extended and enhanced based on the new notions of conditional
equivalence group and normalized class of differential equations. Effective new
techniques are proposed. Using these, we exhaustively describe admissible point
transformations in classes of nonlinear (1+1)-dimensional Schroedinger
equations, in particular, in the class of nonlinear (1+1)-dimensional
Schroedinger equations with modular nonlinearities and potentials and some
subclasses thereof. We then carry out a complete group classification in this
class, representing it as a union of disjoint normalized subclasses and
applying a combination of algebraic and compatibility methods. Moreover, we
introduce the complete classification of (1+2)-dimensional cubic Schroedinger
equations with potentials. The proposed approach can be applied to studying
symmetry properties of a wide range of differential equations.