H. Dye showed that an isomorphism between the (discrete) unitary groups in two factors not of type In is implemented by a linear (or a conjugate linear) * -isomorphism of the factors. If ϕ is an isomorphism between the unitary groups of two unital C * -algebras, it induces a bijective map θϕ between the sets of projections. For certain UHF-algebras, we construct an automorphism ϕ of their unitary group, such that θϕ does not preserve the orthogonality of projections. For a large class of unital finite C * -algebras, we show that θϕ is always an orthoisomorphism. If ϕ is a continuous automorphism of the unitary group of a UHF-algebra A, we show that ϕ is implemented by a linear or a conjugate linear * -automorphism of A.