Changing to port 4500

RFC 4555, Sec. 3.3 says that if both peers support MOBIKE and NAT-T, then they 
must change to port 4500 even if NAT was not detected between them.

But in the design document, RFC 4621, Sec 5.2.3 says that the port change 
should be done immediately after IKE_SA_INIT and before IKE_AUTH. However, 
support for MOBIKE is declared during the IKE_AUTH exchange.

I suppose it's not a big deal, since peers will be listening on port 4500 
anyway. But when should the initiator and responder change ports in the 
scenario where there is no NAT between them?