Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
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I don't know about the Amex 2-4-1 bit, I've never done that.
But for Joker plus GUF2, you need "availability" in merely the end class, but not the paid for class. However being a GGL Redemption / Joker, this availability is for D bucket, which isn't normally too demanding.
Also note that only one GGL booking is allowed per flight, so if someone got in there before you, then you'll be out of luck. This affects SYD, BKK, SIN and MLE, and a bit of CPT.
The only catch that I can see is that Jokers (to the best of my knowledge) are deployed once only, so if you change plans you will lose your Joker and will either need to use another Joker or need plain availability - which late on may not be a problem. Logically you can't have a never-ending Joker so it stands to reason really. This conflicts with G-BOAC's advice above, though I can see cancellation may be treated differently, so hopefully someone else can give their experience with this. GUF2s can definitely be recredited and changed (etc).
The GGL hotline spend a lot of time sorting out these bookings, so I doubt you will have a problem, they have a lot of experience in this area.