Being the one manufacturer, surely a good deal could be done to transition.
Actually locking yourself into one manufacture makes it far harder to get a 'good deal'. The airlines regularly play Boeing and AB off against each other to get the best possible deal. As soon as Boeing or AB decide that an airline isn't really interested in their aircraft and are just using them to play off against the other guy, they loose interest and all your bargaining power goes away.
That's why, except for some VLC operators who operate a single type, you seldom see an operator who has an all Airbus or all Boeing fleet.