A change of type seems really unlikely to an airline like Jet2.
It could be done quite simply. The A320NEO does have 189 passengers in a one class layout, so that’s one objection solved. Then JET2 has 7 bases and they are already allocating their three Boeing types to certain bases. The clearest example is Glasgow which is all B738s. It would therefore be possible to allocate new A320NEOs to specific bases and the mixed fleet would not therefore create any major headaches. Further, with the A320NEO being a 189 seater, it could be swapped for a B738 at any time.
One other advantage is the more spacious cockpit of the A320NEO that would delight Sean Dillon. Doubtless if it comes about it probably means the CEO was thinking of him.