If you want to do something a bit more sophisticated than dumb DGN transfers...
Apparently Bentley recently released "OpenPlant ModelServer V8i" which means you should be able to export ISO 15926 data from Autoplant (I think).
Also apparently pdms has the ability to import 15926 data?
So this could potentially help you out unless dumb copies are OK or you want to remodel the whole project?
You can't do a direct export from AutoPlant to DGN. You will have to acis out all the intelligent graphics in AutoPlant and then open the DWG file in Microstation. Then save it as a DGN file inside of Microstation. That process breaks the intelligent links created with AutoPlant and dumbs all the graphics down to just solids.