duh I'm not saying the NGX is the same as a -200Adv.
In the real world they all came off the same production line at Renton so the option to upgrade or downgrade should be available. There would/should be conditions and costs associated with any order modification.
Realistic? Perhaps. But it's not done in AWS. Because, if it were, through the prescience that we have of knowing that Boeing will launch a better model (within +- 18 months) a person could simply order 100 Boeing 733s, then upgrade them to 734s when that model was launched because they knew it was going to launch within a couple years. It's a mechanism to prevent people from locking up production queues through their ability to see through their 'crystal ball' and see the future that a person wouldn't/shouldn't have.
So, in AWS, you can only upgrade your planes to whatever models were available at the time that you placed your order. So if you ordered 733s even when 734s were available, then you can upgrade to them if you want. But if you were to place a large order for 733s immediately when they were launched, you would not be able to upgrade them to anything that came out in the future.
And upgrading to a whole new "model" -- like a 200Adv to an NGx -- not possible in AWS because they're ran down different production lines and are considered wholly different models. In AWS, unlike real-life, whole different families of the 737 end up in production simultaneously. Sometimes even 200s and 800s get produced simultaneously. But even if they are, you can't upgrade your order between them since they're different production lines.