transfer entitlement, but retain ID

I am currently trying to transfer an entitlement to a new organization, but I would like to keep the same ID as the original. According to the FlexNet Operations Web Services Guide, this is possible by setting the "retainExistingIds" element to true. Unfortunately, a new Id is generated anyway. I also tried to set the newly generated entitlement to use the original one, but I get an error saying it is already in use.

When I try doing this through the website, the ID gets retained correctly. So there must be a way. Can anyone please point me in the right direction?