Let $A \in \mathbb{F}^{n \times n}$, let $X \in \mathbb{F}^{n \times n}$ and let $X=UTU^{*}$ be the complex Schur decomposition, then does the following equality always hold
$$ \| A - UTU^{*} \|^{2}_{F} = \| U^{*} A U - T \|^{2}_{F} $$
with $F$ denoting the Frobenius norm.
Yes. The Frobenius norm is preserved by individual unitary transformations either from the left or from the right as explained in "Approach 1" of this answer to a related question