partial class Examples
{
  public static Result ChangeDimensionStyle(RhinoDoc doc)
  {
    foreach (var rhino_object in doc.Objects.GetObjectList(ObjectType.Annotation))
    {
      var annotation_object = rhino_object as AnnotationObjectBase;
      if (annotation_object == null) continue;

      var annotation = annotation_object.Geometry as AnnotationBase;
      if (annotation == null) continue;

      if (annotation.Index == doc.DimStyles.CurrentDimensionStyleIndex) continue;

      annotation.Index = doc.DimStyles.CurrentDimensionStyleIndex;
      annotation_object.CommitChanges();
    }

    doc.Views.Redraw();

    return Result.Success;
  }
}

from Rhino import *
from Rhino.DocObjects import *
from Rhino.Commands import *
from Rhino.Geometry import *
from scriptcontext import doc

def RunCommand():
    for annotation_object in doc.Objects.GetObjectList(ObjectType.Annotation):
        if not isinstance (annotation_object, AnnotationObjectBase):
            continue

        annotation = annotation_object.Geometry

        if annotation.Index == doc.DimStyles.CurrentDimensionStyleIndex:
            continue

        annotation.Index = doc.DimStyles.CurrentDimensionStyleIndex
        annotation_object.CommitChanges()

    doc.Views.Redraw()
    return Result.Success

if __name__ == "__main__":
    RunCommand()