I am trying to get my head around this: I want to see if I can book a WT+ return fare LHR-LAS. I then want to upgrade this to CW. I do have the miles available but I am unsure how I can be certain that the fare I book will be eligibile for an upgrade.

As I understand, it is possible to MFU from a WT+ discount fare - which it has to be as I cannot afford to purchase a fully flexible WT+ fare.

How can I be sure I can upgrade?

Also - when doing this kind of MFU what is the availability - the same as a redemption, or maybe I can just MFU any flight as long as there is a CW seat free...?