NOT because of the current economy.
In all fairness, AA have over 50 767-300s in the fleet and although they are downgrading a number of routes to free up replacement aircraft for the A300s leaving the fleet, it is a fair bet that the routes being downgraded are the ones whose commercial performance is sub-par.
Put it another way, if the 767-300 was going full every day with profitable high-yield traffic, do you think American would be turning away 40+ pax every flight from Manchester or looking at other sectors in their network which were under-performing and downgrading those to a 757 instead?
They need to downgrade 28 lines of flying to release the A300s for retirement and it would appear that Manchester-ORD being chosen for a downgrade to 757 indicates that the route appears somewhere in the bottom third of the profitability table for AA.
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Routes for Summer 2009:
Air Canada back with a daily YYZ 767 service
Air France launching a daily LYS CRJ service
BA cutback on LGW-MAN from 7 to 5 per day
Flybe having a go at bmi regional on MAN-ABZ, 6 x daily
Continental staying at twice-daily on EWR with 757
Delta staying daily 757 on JFK but at later arrival/departure times than now
Sun-Air adding lunchtime service to Billund
EZY on TFS, FUE, ALC, AGP, HER, GVA, CDG, NCE, PFO, CFU, MLA, SOF
One new route from Ryanair (twice weekly, don't know what)
CSA increasing up to 3 per day
Monarch daily NCE + second daily BCN
Most of this not yet announced.