Train Managers do have the authority to declassify a train but this will only be done in extreme circumstances. As Liam said, only some of the Meridians have sections of their First Class declassified.
I believe I am recalling this correctly, but the 9 coach Meridians were intended for enhanced Leeds-Sheffield-Derby-Leicester-London services but something happened and the plans changed. This meant that the 9 car units sat dormant in sidings for around a year and weren't introduced in to service until 2005, when some of the HSTs went off-lease. In late 2008 and throughout 2009, Stagecoach East Midlands decided to lengthen the existing 4 coach units to 5 car sets by removing 2 coaches from each 9 coach set to form a Meridian fleet of 5 & 7 coach sets. Some of this work involved declassifying First Class compartments, which is why 222007 and 222018 have First Class reading lights in some of their Standard Class accommodation.
Then a short while later, former Hull Trains 4 Coach 222s were taken on by East Midlands Trains for the commencing of the Corby services, and so these 4 units joined the fleet, with 222103 being slightly delayed and having newly built centre carriages added after an unfortunate incident where the unit was dropped from a rig at the depot.
East Midlands Railway (then part of Abellio) then added coaches from some of the seven car sets to the ex Hull Trains sets to make them 5 coaches. Some 7 car units were decreased to 5 cars and their lost coaches donated to increase the 4 coach sets to 5.