How much leeway do I have on ERD?
Unless you measure your spokes and calculate for the hub and ERD, you have no certainty on leeway.
It depends entirely on the spoke length. One build might have spokes just long enough, then a new rim with a 1mm bigger ERD would need new spokes. A different build might have spokes that are just off being too long, then a new rim would need an ERD no smaller than the existing rim.