Given the rate of B733 phaseouts, i would guess its more a case of when, rather than if. They'll want to get as much use out of it when it does come back into service, so my eyes would be the start of the winter season.
I'd also expect a couple more B733's to go as well. Likely candidates are
G-CELE - nearly 31y
G-GDFE - 31y
G-GDFB - 27y
The rest are only 20-22y, so they'd likely stick around a bit longer