SetOptions[NotebookSelection[SelectedNotebook[]], CellStyle->"ItemParagraph"]

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

SelectionMove[SelectedNotebook[], All, Cell]

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

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

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