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If I create a new project, all the target platforms appear. However, if I open an existing project all platforms EXCEPT Windows 64 bit appear. If I right click, the add platform option is grayed out.
Any suggestions appreciated
You need to edit the existing project's dproj file to add/enable the additional platforms, then you can switch between them as needed.
Can you be more explicit about exactly what to change? I have a dozen programs to update.