How much do you want to use the credit or get the money back? Do you want to worry about this again next year or would you prefer to be done with it? I agree that if you're willing to mind the credit for another few years then asking for an extension could be the way to go. After that - sell or let it expire ( especially if they won't do anything for you). I don't think leaving it until the last few weeks is currently a good tactic as shifting regulations and lower staffing levels due to cost cutting will require more time to get through. My only experience with NCL's change management was a lot of time and follow up. Cancellation chicken was a decent tactic when the initial cancellations were going on but since a near term cruise is off the table, I think it is better to deal with it while you have some reasoning to ask for leeway.
I'm currently fine with losing several on-board future deposits if it comes to that and will not be going on a cruise I'm not comfortable with (price, safety, destination) because of sunken cost fallacy. Similar reasoning would apply to cruise cancellations put into more sizable future cruise credits, especially with the restrictions future cruise credits have & not knowing what precautions the line will take. People differ, though.