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I am not aware of Mechanical P&ID. Are you trying to open a P&ID drawing in AutoCAD Mechanical?
Plant Solutions - AEC
Are you sure you're not in Paper Space and the P&ID Lines are in Model Space?
Plant Solutions Engineer
Autodesk Australia / New Zealand
It doesn't matter what version you save it as. P&ID is a custom object which is designed not to be able to modified from another software (for data integrity). You'll need to export it to autocad using 'exporttoautocad' command.
AutoCAD Mechanical is a bit loose on this, you can modify a mechanical object using another software but it will prompt you to reset back to the original drawing once you open it back using AutoCAD Mechanical.
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