Wix MajorUpgrade not working, after changing UpgradeCode GUID & Version - wix

Question

We are using the following code in our Wix Packages.wxs file, as per the documentation we are changing the UpgradeCode GUID & Version number from. 1.0.1.0 to 1.0.2.0 but when we build and try to install the msi package it says older version is still installed and we need to uninstall it to continue.
<Product Id="8B3DFDFF-D894-4A31-AA92-824729385F15" Name="WixCodeBase" Language="1033" Version="1.0.2.0" Manufacturer="Company Name" UpgradeCode="C78D9362-A156-44A2-94D0-AFA19389FFE8">
<Package Id="*" Keywords="Installer" Manufacturer="Company Name" Description="Wix Installer" InstallerVersion="200" Compressed="yes" InstallScope="perMachine" />
<MajorUpgrade Schedule ="afterInstallValidate" AllowDowngrades="no" DowngradeErrorMessage="A newer version of [ProductName] is already installed." />
<Media Id='1' Cabinet='WixPackage.cab' EmbedCab='yes' />
Installer Error

Answer 1

For major upgrades, you change the Product element's Id attribute not the UpgradeCode attribute. In fact, UpgradeCode attribute must remain constant across versions to use the MajorUpgrade element. MSDN has all the details.

Answer 2

I maintain an open source project called IsWiX that provides templates and designers to accelerate the WiX / MSI learning and development process. One of the many things these templates do out of the box is provide proper Major Upgrade support. Consider this code generated by the template:
Code Source
<?xml version="1.0" encoding="UTF-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
<!--
MSIProductVersion is defined in DesktopApplication.wixproj as 0.0.1 for local desktop builds.
You should pass in the MSBuild Property 'MSIProductVersion' to override it during an automated build.
See http://msdn.microsoft.com/en-us/library/windows/desktop/aa370859%28v=vs.85%29.aspx for information on allowable values.
The Product#Id attribute (ProductCode Property) will be a random GUID for each build. This is to support "Major Upgrades" where each install
is a seamless uninstall/reinstall.
-->
<Product Id="*" Name="DesktopApplication" Language="1033" Version="$(var.MSIProductVersion)" Manufacturer="DesktopApplication" UpgradeCode="7220a19b-ed49-4cd1-8002-6af7926441b4">
<Package InstallerVersion="301" Compressed="yes" InstallScope="perMachine" />
<MediaTemplate EmbedCab="yes" />
<!-- Major Upgrade Rule to disallow downgrades -->
<MajorUpgrade DowngradeErrorMessage="A newer version of [ProductName] is already installed." />
<!--Common Launch Condition-->
<!-- Examples at http://wixtoolset.org/documentation/manual/v3/customactions/wixnetfxextension.html -->
<PropertyRef Id="NETFRAMEWORK40FULL"/>
<Condition Message="[ProductName] requires .NET Framework 4.0.">Installed OR NETFRAMEWORK40FULL</Condition>
<!-- Include User Interface Experience -->
<Icon Id="Icon.ico" SourceFile="Resources\Icon.ico"/>
<Property Id="ARPPRODUCTICON" Value="Icon.ico"></Property>
<UIRef Id="UI"/>
<!-- Include Features and Directories Fragment -->
<DirectoryRef Id="INSTALLLOCATION"/>
</Product>
</Wix>
In addition to be documented in the comments, it's also discussed in the tutorials.
In a nutshell, you need to keep UpgradeCode the same and randomize ProductCode.