basic turing machine
Using your convention that the machine EITHER changes symbol OR moves Left/Right, then this is the machine you are looking for:

&lt;q1,a,b,q2&gt;
&lt;q1,b,a,q2&gt;
&lt;q2,a,R,q1&gt;
&lt;q2,b,R,q1&gt;


In other words, the machine alternates between the 'switch the letter' state and the 'move right' state.  [I am adopting the convention that the absence of a transition - say on space - is equivalent to halting; otherwise one needs another transition to cover that]