The problem with the 757/767 fleet is that it has become a very 'senior' fleet due to internal movement (
) onto the 'mixed' fleet.
If anyone drops out of a conversion course last minute then they might well offer it to a DEP as in the case of my mate. The only fly in the ointment with that is that you will be at the bottom of a long seniority list for a very long time and therefore have little control over your rostering (blindline hell).
As with everything BA seniority is key so don't hold out waiting for that fleet when another is available. You can always transfer later.