According to the literature, when monoclinic crystal structure transforms to triclinic, the following formulaes, regarding their lattice parameters are valid:
a(T) = 1/2 a(M) - 1/2 b(M)
b(T) = 1/2 a(M) + 1/2 b(M)
c(T) = c(M)
Where T and M stand for Triclinic and Monoclinic, respectively...