LowercaseDocumentFilter.cs

using System.Collections.Generic;
using System.Web.Http.Description;
using Swashbuckle.Swagger;

namespace BoundedContext.Web.Swagger
{
	public class LowercaseDocumentFilter : IDocumentFilter
	{
		public void Apply(SwaggerDocument swaggerDoc, SchemaRegistry schemaRegistry, IApiExplorer apiExplorer)
		{
			////////	PATHS
			
			var paths = swaggerDoc.paths;

			//	generate the new keys
			var newPaths = new Dictionary<string, PathItem>();
			var removeKeys = new List<string>();
			foreach (var path in paths)
			{
				var newKey = path.Key.ToLower();
				if (newKey != path.Key)
				{
					removeKeys.Add(path.Key);
					newPaths.Add(newKey, path.Value);
				}
			}

			//	add the new keys
			foreach (var path in newPaths)
			{
				swaggerDoc.paths.Add(path.Key, path.Value);
			}

			//	remove the old keys
			foreach (var key in removeKeys)
			{
				swaggerDoc.paths.Remove(key);
			}
		}
	}
}

You should check your dictionary for repeated key before adding to avoid runtime issues. Other that that, the extension is great!

One more case.
For instance, if we have an endpoint with parameter such as /api/v1/orders/{orderId} it changing to /api/v1/orders/{orderid}.
And when we try to make request instead of
curl -X GET "http://127.0.0.1:9001/api/v1/orders/1" -H "accept: application/json"
we receive
curl -X GET "http://127.0.0.1:9001/api/v1/orders/{orderid}" -H "accept: application/json"