The 737s are already here and as time goes on the the 'plan', as far as I know is for ThomsonFly to operate only 737s and 787s. Recent rumour is that two 762s and one 763 going this year.
The choice is between the 787 and A350 but the rest of TUI have Boeings. I can't see the 737s being replaced now with a smaller airbus.