I was looking for
Sec. 204.5 (e) (204.5): to interpret what it means and found this thread where someone already did it:
http://immigrationvoice.org/forum/showthread.php?t=912
Looks like those sections doesn't say anything about employer withdrawing the I-140. And thats where the memo explains the procedure. Also, from there another attorney agrees that PD can be ported.
BTW, only when you file I-485, you can use the old I-140 to port the PD. My take is, since only now we have retrogression, there is interest in using this provision (otherwise one would have filed 485 already and used the AC21). And those interested might not file I-485 yet due to retrogression, so we might not yet have a previous example to follow. Again this is just my opinion...