The solar sail is one of the most promising space exploration systems due to its theoretically infinite specific impulse achieved through solar radiation pressure (SRP). Recently, researchers have proposed "transformable spacecraft" capable of actively reconfiguring their body configurations using actuatable joints. Transformable spacecraft, if used similarly to solar sails, are expected to significantly enhance orbit and attitude control capabilities owing to their high redundancy in control degrees of freedom. However, controlling them becomes challenging due to their large number of inputs, leading previous researchers to impose strong constraints to limit their potential control capabilities. This study focuses on novel attitude control techniques for transformable spacecraft under SRP. We developed two methods, namely, joint angle optimization to obtain arbitrary SRP force and torque, and momentum damping control driven by joint angle actuation. Our proposed methods are formulated in a general manner and can be applied to any transformable spacecraft comprising front faces that can predominantly receive the SRP on each body. The validity of our proposed method is confirmed through numerical simulations. Our study contributes to making most of the high control redundancy of transformable spacecraft without the need for expendable propellants, thus significantly enhancing the orbit and attitude control capabilities.