Jet2 will not be No 2 airline at STN next year, they will have approx 20 flights a day where as EZY will have 30+. Although Jet2 will have 3 more aircraft they do 2 rotations where as Ezy do 3.
It will be interesting in the future as EZY's key strategy is being the No1 or No2 position at any airport they are based at. Hence why they have given up on HAM.
No airframes will move from STN to SEN, they're totally different markets as EZY has realised by the trial and error of routes from SEN that have come and gone. They've already made the mistake of moving a 4th. They have realised SEN is a bucket and spade niche with the ability to achieve really decent yields on those summer routes and some low frequency city destinations compared to the main London airports. STN is key to EZY's domestic strategy with routes fit around those schedules. With long serving 'random' routes like LJU, BIO, OVD still achieving good yields.