partial class Examples
{
  public static Result RenameBlock(RhinoDoc doc)
  {
    // Get the name of the insance definition to rename
    var instance_definition_name = "";
    var rc = RhinoGet.GetString("Name of block to rename", true, ref instance_definition_name);
    if (rc != Result.Success)
      return rc;
    if (string.IsNullOrWhiteSpace(instance_definition_name))
      return Result.Nothing;

    // Verify instance definition exists
    var instance_definition = doc.InstanceDefinitions.Find(instance_definition_name, true);
    if (instance_definition == null) {
      RhinoApp.WriteLine("Block \"{0}\" not found.", instance_definition_name);
      return Result.Nothing;
    }

    // Verify instance definition is rename-able
    if (instance_definition.IsDeleted || instance_definition.IsReference) {
      RhinoApp.WriteLine("Unable to rename block \"{0}\".", instance_definition_name);
      return Result.Nothing;
    }

    // Get the new instance definition name
    string instance_definition_new_name = "";
    rc = RhinoGet.GetString("Name of block to rename", true, ref instance_definition_new_name);
    if (rc != Result.Success)
      return rc;
    if (string.IsNullOrWhiteSpace(instance_definition_new_name))
      return Result.Nothing;

    // Verify the new instance definition name is not already in use
    var existing_instance_definition = doc.InstanceDefinitions.Find(instance_definition_new_name, true);
    if (existing_instance_definition != null && !existing_instance_definition.IsDeleted) {
      RhinoApp.WriteLine("Block \"{0}\" already exists.", existing_instance_definition);
      return Result.Nothing;
    }

    // change the block name
    if (!doc.InstanceDefinitions.Modify(instance_definition.Index, instance_definition_new_name, instance_definition.Description, true)) {
      RhinoApp.WriteLine("Could not rename {0} to {1}", instance_definition.Name, instance_definition_new_name);
      return Result.Failure;
    }

    return Result.Success;
  }
}

import rhinoscriptsyntax as rs
from scriptcontext import doc

def Rename():
    blockName = rs.GetString("block to rename")
    instanceDefinition = doc.InstanceDefinitions.Find(blockName, True)
    if not instanceDefinition:
        print("{0} block does not exist".format(blockName))
        return

    newName = rs.GetString("new name")
    instanceDefinition = doc.InstanceDefinitions.Find(newName, True)
    if instanceDefinition:
        print("the name '{0}' is already taken by another block".format(newName))
        return

    rs.RenameBlock(blockName, newName)

if __name__ == "__main__":
    Rename()