YourProvider.cs

using System;
using System.Linq;
using Orchard;
using Orchard.DisplayManagement.Descriptors;
using Orchard.Environment.Extensions;

[OrchardFeature("The.Name.of.a.Feature")]
namespace YourProvider.Providers {
    public class YourPrivider : IShapeTableProvider {
        public void Discover(ShapeTableBuilder builder) {
            builder.Describe("Content").OnDisplaying(
                displaying => {
                      if (somecondition, path or shapename or something) {
                        displaying.ShapeMetadata.Alternates.Add("HereBeAShape");
                      }
                    }
              });
      }
}