In that case your outbound and inbound will be on different fare basis (EK construct fares after return/one-way on an outbound and inbound basis separately) - in the case of a connection at DXB not O&D there, both sectors on an inbound or outbound sectors XXX-DXB-YYY will have the same fare basis (i.e. XXX-DXB and DXB-YYY) but the inbound and outbound can be different. (YYY-DXB and DXB-XXX will be the same though. But different to the reverse route). So you may be able to upgrade one or both of the W basis sectors but not the other saver fare sectors assuming they are the really cheap savers that can't be upgraded before check-in.