It is not perfect, but most airlines will try not to swap plane models as it breaks all the seating assignments. I think the older Boeing planes through 737-90

It is not perfect, but most airlines will try not to swap plane models as it breaks all the seating assignments. I think the older Boeing planes through 737-900 are as safe as ever. 737-MAX+ and 787 seem to be where the design process went awry. Even then, there seem to be known workarounds to the bugs. For what it's worth, I still fly on them all, I just remember to tell my family I love them before takeoff when the safety card has MAX on the label.