Set the style of a cell

I don't know if any of them can be callled documented but I know three ways to do this:

mentioned above:

FrontEndToken way:

(
 SelectionMove[#, All, Cell]; 
 FrontEndTokenExecute[EvaluationNotebook[], "Style", "Title"]
) & /@ Cells[CellStyle -> "Section"]

or

SelectionSetStyle way:

(
 SelectionMove[#, All, Cell];
 MathLink`CallFrontEnd[FrontEnd`SelectionSetStyle[#, "Title"]]
   ) & /@ Cells[CellStyle -> "Section"]

or (which is implemented with code from above)

CellStyleNames way:

(
 SelectionMove[#, All, Cell];
 Experimental`CellStyleNames[#] = "Title";
) & /@ Cells[CellStyle -> "Section"]

Maybe this button may help you:

Button["ItemParagraph", FrontEndExecute@FrontEndToken[SelectedNotebook[], "Style", "ItemParagraph"]]

The whole cell may be selected with:

SelectionMove[SelectedNotebook[], All, Cell]