calling WebDynpros in ECC 6 not from the local portal

Hey,

we are having a portal in place pointing to another system with ERP2004 end self developed WebDynpro applications in HCM. Now we want to switch to ECC 6 and so we need to have a local portal in this system - correct?

Would we be able to use the "old" main portal to call this local HCM WebDynpros as we did before? Is this supported by SAP? or do we have to use the local portal as a producer portal and connect the the main portal as an FPN?

What are the arguments to use FPN functionallity instead of calling WebDynpros directly?

Thanks in advance and best regards

Michael