It would give them the opportunity to open some thinner routes out of Cork and Belfast and on some of the low volume routes out of Dublin, it could allow them to open multiple daily flights instead of a single one, which would be needed if they are serious about turning Dublin into a hub for transatlantic connections.
But, with oil above $90 a barrel, a jet with fewer than 100 people will struggle economically, even before fleet uniformity problems are taken into account. And as Mark implies, financial markets may flinch at seeing Aer Lingus moving into what looks like a higher cost model.