Transition Matrices between Shifted $t$-Schur Bases and Cyclotomic Schur $Q$-Positivity

For a strict partition $\lambda$, let $\mathcal Q_\lambda(X;t)=Q_\lambda[X-tX]$ be the shifted $t$-Schur function arising from the modified Greaves--Jing--Zhu operator on the odd power-sum ring. We study transition matrices between the shifted bases with parameters $t$ and $s$. The relative scaling operator is diagonal in the odd power-sum basis, leading to explicit spectral data, determinant and trace formulas, weighted symmetry, a spin-character formula, and a transition Cauchy identity. For the cyclotomic specialization $C_{\lambda\mu}^{[M]}(t)=C_{\lambda\mu}(t^M,t)$, the relative operator becomes plethystic substitution by $1+t+\cdots+t^{M-1}$. We prove Schur $Q$-positivity and reciprocity, derive factorization and root-of-unity rank formulas, and give an exact computation method. For $M=2$, all one-row transitions are computed explicitly, and the nonzero coefficients are unimodal.