Hi I'm cc for TUI; for CC there is practically no difference between the MAX and NG other than a couple of safety equipment location differences which can be covered in an online training module. Seating layout is identical. For flight crew the two variants are totally interchangeable once some additional training on the Max has been completed. I'm not 100% but presume max a/c are being plotted into regional programmes in order to give flight crew at based regional stations opportunities to get increased exposure to it.