how to change from per user to all user installation?

Question

For our msi , we did not mention allusers property in existing releases. so by default it went as per user.

Our customers installed the application using an admin us

Answer 1

Unfortunately, the Windows installer does not support upgrading "across contexts". As you found a per-machine package cannot upgrade a per-user package and a per-user package cannot upgraded a per-machine package.

You could write an executable that finds the old .msi and uninstalls them using APIs like ::MsiEnumRelatedProducts() to find the products and then ::MsiConfigureProduct() to remove them. However, without the user account, I'm not sure you'll be able to see the per-user installed product.