Can confirm the experience of the other posters here - at least for usage of corporate upgrade vouchers. There is no certainty even up until boarding.

Old vouchers being used in 2007 (and maybe still 2008), involved calling up CX and voucher has been linked to specific flight with CX by asking for availability and them recording the request. Can't remember any more whether it was a 100% confirmation, but think that they remarked that they still reserved the right to cancel any time. Have used something like 3-4 vouchers.

Then from 2009 onwards (if I recall correctly), the vouchers could not be linked to an itinerary in advance any more by asking CX for seat availability. They had to be presented when physically checking in at the counter by asking for availability.

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Tried to use it on HKG-LHR which was obviously full. Got denied the first attempt. Second attempt for similar trip this year got denied as well. But spoke this time with check-in agent and explained politely my situation with having to directly continue LHR-ZRH and then having meeting right after arrival. Got then upgraded when boarding. Suppose that she called a supervisor to get approval after I left check-in.

It's anyone's guess what exactly happened, but this may give some indication on priority: probably not very high. I think as well that it could help to arrive as early as possible before op-up system kicks in using tier and BN hierarchy.