Say I have
$$\dfrac{1}{\sqrt{2}}\bigl(|1\rangle|221\rangle|0\rangle + |3\rangle|73\rangle|2\rangle\bigr).$$
How can I change that into
$$\dfrac{1}{\sqrt{2}}\bigl(|1\rangle|221\rangle|1\rangle + |3\rangle|73\rangle|2\rangle\bigr)?$$
Say I have
$$\dfrac{1}{\sqrt{2}}\bigl(|1\rangle|221\rangle|0\rangle + |3\rangle|73\rangle|2\rangle\bigr).$$
How can I change that into
$$\dfrac{1}{\sqrt{2}}\bigl(|1\rangle|221\rangle|1\rangle + |3\rangle|73\rangle|2\rangle\bigr)?$$
Assuming that your last spin is of dimension 3, why not just apply the unitary $$ \left(\begin{array}{ccc} 0 & 1 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 1 \end{array}\right)? $$
The first register the way you write it is about two qubits if not more. In the state, it is either 01 (1) or 11 (3). Then I would just use a X/NOT controlled by the two qubits of the register and only applied when the control is 01. This is done by Toffoli where you use X gate in between the first qubit so the control is applied when it is 0.