diff --git a/msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.props b/msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.props
index db23a7978d1c..410d78dad91d 100644
--- a/msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.props
+++ b/msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.props
@@ -26,6 +26,20 @@ Copyright (C) 2013-2014 Xamarin. All rights reserved.
 		<_XamarinCommonPropsHasBeenImported>true</_XamarinCommonPropsHasBeenImported>
 	</PropertyGroup>
 
+
+	<!-- Story-time! MigrateToNewXMIdentifier is special because it un-does a lie we started telling from the 
+	beginning of Full (called Xamarin.Mac 4.5 back then). Back then, we released Modern (called Mobile) and 
+	when many customers upgraded from Classic they broke in terrible ways as none of their nugets worked. 
+	No nugets in the wild had heard of the Xamarin.Mac TFI and thus didn't support it. To "fix" this use case,
+	we created Full (XM 4.5) and _lied_ with our TFI claiming to be .NETFramework. Well we were close enough
+	for a vast majority of packages to work. However, this made creating Full specific package flavors impossible.
+	Now that we're getting "real" nuget support, we need to stop lying without breaking the world. We're starting by
+	allowing "opt-in" MigrateToNewXMIdentifier which will swap your declared TFI to the new hotness.
+	-->
+	<PropertyGroup Condition=" '$(MigrateToNewXMIdentifier)' == 'true' And '$(TargetFrameworkIdentifier)' != 'Xamarin.Mac' And '$(UseXamMacFullFramework)' == 'true' ">
+		<TargetFrameworkIdentifier>Xamarin.Mac.NET</TargetFrameworkIdentifier>
+	</PropertyGroup>
+
 	<PropertyGroup>
 		<IsXBuild Condition="'$(MSBuildRuntimeVersion)' == ''">true</IsXBuild>
 		<IsXBuild Condition="'$(MSBuildRuntimeVersion)' != ''">false</IsXBuild>
diff --git a/tests/mmptest/mmptest.csproj b/tests/mmptest/mmptest.csproj
index d2efabf9332b..76dfeb5f2de1 100644
--- a/tests/mmptest/mmptest.csproj
+++ b/tests/mmptest/mmptest.csproj
@@ -113,6 +113,7 @@
     <Compile Include="..\..\tools\mmp\SdkVersions.cs">
       <Link>SdkVersions.cs</Link>
     </Compile>
+    <Compile Include="src\TargetFrameworkMutateTests.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="Info.plist" />
diff --git a/tests/mmptest/src/TargetFrameworkMutateTests.cs b/tests/mmptest/src/TargetFrameworkMutateTests.cs
new file mode 100644
index 000000000000..74912a241e4c
--- /dev/null
+++ b/tests/mmptest/src/TargetFrameworkMutateTests.cs
@@ -0,0 +1,58 @@
+using System;
+using System.Linq;
+using NUnit.Framework;
+using Xamarin.Utils;
+
+namespace Xamarin.MMP.Tests
+{
+	[TestFixture]
+	public class TargetFrameworkMutateTests
+	{
+		const string MigrateCSProjTag = "<MigrateToNewXMIdentifier>true</MigrateToNewXMIdentifier>";
+
+		public bool MatchesTFI (string expected, string buildOutput)
+		{
+			string tfiLine = buildOutput.SplitLines ().FirstOrDefault (x => x.StartsWith ("TargetFrameworkIdentifier =", StringComparison.Ordinal));
+			if (tfiLine == null)
+				return false;
+			return tfiLine.Contains (expected);
+		}
+
+		[TestCase (true)]
+		[TestCase (false)]
+		public void ShouldNotMutateWithoutOptInFlag (bool xm45)
+		{
+			MMPTests.RunMMPTest (tmpDir => {
+				TI.UnifiedTestConfig test = new TI.UnifiedTestConfig (tmpDir) {
+					XM45 = xm45
+				};
+				var buildOutput = TI.TestUnifiedExecutable (test).BuildOutput;
+				string standardTFI = xm45 ? ".NETFramework" : "Xamarin.Mac";
+				Assert.True (MatchesTFI (standardTFI, buildOutput), $"Build did not have expected TFI: {TI.PrintRedirectIfLong (buildOutput)}");
+			});
+		}
+
+		[Test]
+		public void ShouldNotMutateModernWithOptInFlag ()
+		{
+			MMPTests.RunMMPTest (tmpDir => {
+				TI.UnifiedTestConfig test = new TI.UnifiedTestConfig (tmpDir) { CSProjConfig = MigrateCSProjTag };
+				var buildOutput = TI.TestUnifiedExecutable (test).BuildOutput;
+				Assert.True (MatchesTFI ("Xamarin.Mac", buildOutput), $"Build did not have expected TFI: {TI.PrintRedirectIfLong (buildOutput)}");
+			});
+		}
+
+		[Test]
+		public void ShouldMutateFullWithOptInFlag ()
+		{
+			MMPTests.RunMMPTest (tmpDir => {
+				TI.UnifiedTestConfig test = new TI.UnifiedTestConfig (tmpDir) {
+					XM45 = true,
+					CSProjConfig = MigrateCSProjTag
+				};
+				var buildOutput = TI.TestUnifiedExecutable (test).BuildOutput;
+				Assert.True (MatchesTFI ("Xamarin.Mac.NET", buildOutput), $"Build did not have expected TFI: {TI.PrintRedirectIfLong (buildOutput)}");
+			});
+		}
+	}
+}