Worst case? You have to spend a minute or two on some graphs replacing a few custom nodes with updated versions. It sucks but it’s an opportunity to review your old work and streamline it some now that you know more than you previously had written.
Better question is what is the consequence if you don’t update? Well, considering development isn’t stopping, you miss out on all the great new stuff: increased performance, new custom nodes which do things you currently do by hand or with significant difficulty programmatically, and the like. Basically you’re handcuffing yourself to the features you have now but no more as a result of stopping updates. Trust me the new features will be worth it.
Also, when you install the newer Revit versions you’ll be installing a new Dynamo version anyway, and I’d guess that you’d like to have the tools you use in your current version working on the new version anyway. Best to get that up and running before you need it in a real project.