Isospectral transformation of ODE

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Is there a transformation of coefficients of differential operators w/periodic coefficients on the real line $a_n(x+2\pi)=a_n(x)$, that preserves the eigenvalues of their monodromy matrices? $$Df(x)=\sum_n a_n(x)f^{(n)}(x)=0,$$ $$M_D:(f^{(n)}(0))\rightarrow (f^{(n)}(2\pi)).$$ In particular to constant coefficients? Is this a part of Floquet theory?