Wix, custom dialog when previous version exists

I'm making installation for my application with WiX toolset. I want to show custom dialog if previous version found.

Now I don't know how to check if there is previous version and show this dialog only in that case? Here is my code.

CustomDialogUI.wxs :

 <?xml version="1.0" encoding="utf-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
  <Fragment>
    <UI>
  <Dialog Id="OldVersionDlg" Width="260" Height="85" Title="[ProductName] Setup" NoMinimize="yes">
    <Control Id="No" Type="PushButton" X="132" Y="57" Width="56" Height="17"
      Default="yes" Cancel="yes" Text="No">
      <Publish Event="EndDialog" Value="Exit">1</Publish>
    </Control>
    <Control Id="Yes" Type="PushButton" X="72" Y="57" Width="56" Height="17" Text="Yes">
      <Publish Event="EndDialog" Value="Return">1</Publish>
    </Control>
    <Control Id="Text" Type="Text" X="5" Y="15" Width="250" Height="30">
      <Text>A previous version of [ProductName] is currently installed. By continuing the installation this version will be uninstalled. Do you want to continue?</Text>
    </Control>

  <Control Id="Back" Type="PushButton" X="180" Y="243" Width="56" Height="17" Text="&amp;Back">
    <Publish Event="NewDialog" Value="LicenseAgreementDlg">1</Publish>
  </Control>
  <Control Id="Next" Type="PushButton" X="236" Y="243" Width="56" Height="17" Default="yes" Text="&amp;Next">
    <Publish Event="ValidateProductID" Value="0">1</Publish>
    <Publish Event="SpawnWaitDialog" Value="WaitForCostingDlg">CostingComplete = 1</Publish>
    <Publish Event="NewDialog" Value="SetupTypeDlg">ProductID</Publish>
  </Control>
  <Control Id="Cancel" Type="PushButton" X="304" Y="243" Width="56" Height="17" Cancel="yes" Text="Cancel">
    <Publish Event="SpawnDialog" Value="CancelDlg">1</Publish>
  </Control>

  <Control Id="BannerLine" Type="Line" X="0" Y="50" Width="370" Height="0" />
  </Dialog>      
</UI>

With this I have created my dialog as I wanted and inserted into the original chain of dialogs.

And in my Project.wxs

<UI Id="MyWixUI_Mondo">
      <UIRef Id="WixUI_Mondo" />
      <UIRef Id="WixUI_ErrorProgressText" />

  <DialogRef Id="OldVersionDlg" />

  <Publish Dialog="LicenseAgreementDlg" Control="Next" Event="NewDialog" Value="OldVersionDlg" Order="3">LicenseAccepted = "1"</Publish>
  <Publish Dialog="SetupTypeDlg" Control="Back" Event="NewDialog" Value="OldVersionDlg">1</Publish>
</UI>

With this I get my OldVErsionDlg everytime, and I don't know how to make him show ONLY if previous version exists.

I have added to Project.wxs

<Property Id="PREVIOUSVERSIONSINSTALLED" Secure="yes" />
<Upgrade Id="{80EE38CE-1A3B-445F-8CC1-31B32AA77715}">
  <UpgradeVersion Minimum="1.0.0.0" Maximum="9.0.0.0"
                  Property="PREVIOUSVERSIONSINSTALLED"
                  IncludeMinimum="yes" IncludeMaximum="no" />
</Upgrade>

and tried with:

 <UI Id="MyWixUI_Mondo">
  <UIRef Id="WixUI_Mondo" />
  <UIRef Id="WixUI_ErrorProgressText" />

  <DialogRef Id="OldVersionDlg" />

  <Publish Dialog="LicenseAgreementDlg" Control="Next" Event="NewDialog" Value="OldVersionDlg" Order="3">LicenseAccepted = "1" AND PREVIOUSVERSIONSINSTALLED</Publish>
  <Publish Dialog="SetupTypeDlg" Control="Back" Event="NewDialog" Value="OldVersionDlg">1</Publish>
</UI>

No luck with that.

Next / Back Button Override: I am no GUI expert, in fact it is a neglected area (GUI is always suppressed for corporate deployment), but I believe something like this can work - redefining the Next and Back buttons - 3 dialogs involved:

<!-- 1. OldVersionDlg showing -->
<Publish Dialog="LicenseAgreementDlg" Control="Next" Event="NewDialog" 
         Value="OldVersionDlg">LicenseAccepted = "1" AND PREVIOUSVERSIONSINSTALLED</Publish>

<Publish Dialog="SetupTypeDlg" Control="Back" Event="NewDialog" 
         Value="OldVersionDlg">PREVIOUSVERSIONSINSTALLED</Publish>

<!-- 2. OldVersionDlg dialog not showing -->
<Publish Dialog="LicenseAgreementDlg" Control="Next" Event="NewDialog" 
         Value="SetupTypeDlg">LicenseAccepted = "1" AND NOT PREVIOUSVERSIONSINSTALLED</Publish>

<Publish Dialog="SetupTypeDlg" Control="Back" Event="NewDialog" 
         Value="LicenseAgreementDlg">NOT PREVIOUSVERSIONSINSTALLED</Publish>

Actual Dialog Sequences:

LicenseAgreementDlg <-> OldVersionDlg <-> SetupTypeDlg
LicenseAgreementDlg <-> SetupTypeDlg

To be sure, remember to test in all installation modes: install, upgrade, repair, modify, patching, uninstall (and there are stranger shores such as resumed installs - rarely seen).

In closing: is this feature really necessary? I find that features like these often cause more bugs than benefits, but that is just a personal opinion. In fact I wrote an answer a long time ago on how to implement such an upgrade check without using setup dialogs.

Some Links: