In class switching in the secondary antibody response, the most common antibody switch is from

I believe that the most common antibody switch is from IgM to IgG.
Class switching occurs by a mechanism called the class switch recombination binding. It is a biological mechanism that allows the class of antibody produced by an activated B cell to change during a  process known as isotype or class switching. The mechanism changes a B cell's production of antibodies from one form  to another, such as from the isotype IgM to the isotype IgG.