A plugin which allows editors to easily move categories within Episerver (https://jakejon.es/blog/a-plugin-to-move-categories-in-episerver)

MoveCategories.aspx

<%@ Page Language="c#" CodeBehind="MoveCategories.aspx.cs" AutoEventWireup="False" Inherits="AlloyDemoKit.AddOns.MoveCategories.MoveCategories" Title="Move Categories" %>
<%@ Register TagPrefix="EPiServerUI" Namespace="EPiServer.UI.WebControls" Assembly="EPiServer.UI, Version=11.23.0.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7" %>

<asp:content contentplaceholderid="MainRegion" runat="server">
	<asp:Panel runat="server" CssClass="EP-systemMessage" ID="errorPanel" Visible="false">
		<asp:Literal runat="server" ID="errorText" />
	</asp:Panel>

	<div class="epi-formArea epi-paddingVertical-small">
		<div class="epi-size15">
			<div>
				<asp:Label AssociatedControlID="sourceList" Text="Category" runat="server" />
				<asp:DropDownList ID="sourceList" runat="server"/>
			</div>
			<div>
				<asp:Label AssociatedControlID="destinationList" Text="Destination" runat="server" />
				<asp:DropDownList ID="destinationList" runat="server"/>
			</div>
			<div class="epi-buttonContainer">
				<EPiServerUI:ToolButton id="saveButton" DisablePageLeaveCheck="true" OnClick="Move_Click" runat="server" SkinID="Save" text="Move" ToolTip="Move the specified category" />
			</div>
		</div>
	</div>
</asp:content>

MoveCategories.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Web.UI.WebControls;
using EPiServer;
using EPiServer.DataAbstraction;
using EPiServer.PlugIn;
using EPiServer.Security;
using EPiServer.ServiceLocation;
using EPiServer.Shell.WebForms;

namespace AlloyDemoKit.AddOns.MoveCategories
{
	[GuiPlugIn(
		DisplayName = "Move Categories",
		Description = "Move Categories",
		RequiredAccess = AccessLevel.Administer,
		Area = PlugInArea.AdminMenu,
		Url = "~/AddOns/MoveCategories/MoveCategories.aspx")]
	public partial class MoveCategories : WebFormsBase
	{
		private readonly Injected<CategoryRepository> _categoryRepository = default(Injected<CategoryRepository>);

		protected override void OnPreInit(EventArgs e)
		{
			base.OnPreInit(e);

			MasterPageFile = UriSupport.ResolveUrlFromUIBySettings("MasterPages/EPiServerUI.master");
			SystemMessageContainer.Heading = "Move Categories";
			SystemMessageContainer.Description = "Move a category (and it's descendants) to anywhere within the category tree.";
		}

		protected override void OnLoad(EventArgs e)
		{
 			if (!PrincipalInfo.HasAdminAccess)
 			{
				Response.Clear();
				Response.StatusCode = (int)HttpStatusCode.Unauthorized;
				return;
			}

 			base.OnLoad(e);

			if (!IsPostBack) {
				Setup();
			}
		}

		private void Setup()
		{
			// Hide any errors
			errorPanel.Visible = false;
			errorText.Text = null;

			var root = _categoryRepository.Service.GetRoot();

			var items = new List<ListItem>();
			GetCategories(root, items, 0);

			sourceList.DataSource =
				items.Where(x => !x.Value.Equals(root.ID.ToString(), StringComparison.OrdinalIgnoreCase));
			sourceList.DataValueField = nameof(ListItem.Value);
			sourceList.DataTextField = nameof(ListItem.Text);
			sourceList.DataBind();

			destinationList.DataSource = items;
			destinationList.DataValueField = nameof(ListItem.Value);
			destinationList.DataTextField = nameof(ListItem.Text);
			destinationList.DataBind();
		}

		private static void GetCategories(Category category, ICollection<ListItem> items, int depth)
		{
			items.Add(new ListItem
			{
				Text = $"{new StringBuilder().Insert(0, "⁠—", depth)}{category.Description ?? category.Name}",
				Value = category.ID.ToString()
			});

			if (category.Categories == null)
			{
				return;
			}

			foreach (var child in category.Categories)
			{
				GetCategories(child, items, depth + 1);
			}
		}

		private void ShowError(string message)
		{
			Setup();
			errorPanel.Visible = true;
			errorText.Text = message;
		}

		protected void Move_Click(object sender, EventArgs e)
		{
			var source = Request.Form[(string) sourceList.UniqueID];
			var destination = Request.Form[(string) destinationList.UniqueID];

			if (string.IsNullOrEmpty(source))
			{
				ShowError("No category selected.");
				return;
			}

			if (string.IsNullOrEmpty(destination))
			{
				ShowError("No destination category selected.");
				return;
			}

			int sourceId;
			int destinationId;

			if (!int.TryParse(source, out sourceId))
			{
				ShowError("Invalid category selected.");
				return;
			}

			if (!int.TryParse(destination, out destinationId))
			{
				ShowError("Invalid destination category selected.");
				return;
			}

			var category = _categoryRepository.Service.Get(sourceId);

			if (category == null)
			{
				ShowError("Could not retrieve the specified category.");
				return;
			}

			if (category.Parent.ID == destinationId)
			{
				ShowError("Category already lives under the specified destination.");
				return;
			}

			category = category.CreateWritableClone();

			// Update the parent
			category.Parent = _categoryRepository.Service.Get(destinationId);
			_categoryRepository.Service.Save(category);

			Setup();
		}
	}
}

MoveCategories.aspx.designer.cs

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a tool.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated. 
// </auto-generated>
//------------------------------------------------------------------------------

namespace AlloyDemoKit.AddOns.MoveCategories {
    
    
    public partial class MoveCategories {
        
        /// <summary>
        /// errorPanel control.
        /// </summary>
        /// <remarks>
        /// Auto-generated field.
        /// To modify move field declaration from designer file to code-behind file.
        /// </remarks>
        protected global::System.Web.UI.WebControls.Panel errorPanel;
        
        /// <summary>
        /// errorText control.
        /// </summary>
        /// <remarks>
        /// Auto-generated field.
        /// To modify move field declaration from designer file to code-behind file.
        /// </remarks>
        protected global::System.Web.UI.WebControls.Literal errorText;
        
        /// <summary>
        /// sourceList control.
        /// </summary>
        /// <remarks>
        /// Auto-generated field.
        /// To modify move field declaration from designer file to code-behind file.
        /// </remarks>
        protected global::System.Web.UI.WebControls.DropDownList sourceList;
        
        /// <summary>
        /// destinationList control.
        /// </summary>
        /// <remarks>
        /// Auto-generated field.
        /// To modify move field declaration from designer file to code-behind file.
        /// </remarks>
        protected global::System.Web.UI.WebControls.DropDownList destinationList;
        
        /// <summary>
        /// saveButton control.
        /// </summary>
        /// <remarks>
        /// Auto-generated field.
        /// To modify move field declaration from designer file to code-behind file.
        /// </remarks>
        protected global::EPiServer.UI.WebControls.ToolButton saveButton;
    }
}