CormacL
But I bet on the airline you are talking about, the first 10 feet of the cabin is taken up with catering stowages, Club Class wardrobes, etc. etc. (take those out and you've gained 2 rows = 12 seats straight away), and very similar at the rear.
Take out the elements you don't need for the eJ service, re-jig the galley/toilet configurations slightly, and a few extra rows will fit no problem. The seat pitch on the eJ A319's will be the same (or marginally better in some areas) than the 737's.