I'm not an expert, but my recent successful experience of upgrading one segment of a two-leg journey (LHR-SIN-SYD) was not reliant on Avios availability being available on both legs to be able to upgrade only one.
In other words, when I called to upgrade SIN-SYD, there were no seats available LHR-SIN but that did not prevent me upgrading the second leg.
Last edited by paulieuk; Mar 24, 2015 at 9:50 am