Training for different models of the same aircraft type is called differences training. The extent of the required differences training varies with the extent of the differences between the models.

The differences training between the 737 NG and 737 MAX is a CBT course which, IIRC, took a couple of hours and a session in a MAX simulator.

The differences between the 737-8 and 737-9 are very minor. All I can think of, off the top of my head, is the different weight limits and those are not memory items. The length and turning clearance requirements are also different but that's very similar to the 737-800 vs. 737-900.

The 737-8s are being delivered with a head-up-display on the Captain's side and the existing aircraft will eventually be retrofitted. They are currently disabled as we have not been trained on them yet. They will also have to add the authorization to use the HUD into our operations specifications. I don't know what will be involved in the HUD training. I'd guess that there will be a CBT module then some simulator training which will be incorporated into our normal 9-month sim training cycle.

The 737-8s are also the first aircraft with the new AVOD installed. The new AVOD system comes with the capability of having Channel 9 but we don't yet have procedures for that so it's not yet being used. I think all the hardware and software is in place, we just need to flip the switch to activate it once the procedures are established. So far, the only training we've received on this is a pilot bulletin saying to keep the switch off. There will likely be another pilot bulletin when we turn it on with the new procedures and that will then be incorporated into the next flight manual revision.