Hi again Tim,
For the Substance Designer's workflow, we don't talk about opening your Painter's file with Designer, we're talking about trasferring your current work onto a new model made for the legacy workflow.
If the UVs stay the same, you could also simply export your current maps as bitmap and reimport them in the new Legacy project. But no matter what, the layer stack won't be sent to Photoshop without a Legacy workflow.
Best regards,