There should be little to no impact on existing drivers because each of these interfaces gets an entirely new driver which 'slots right in' thanks to how Linux's drivers work.
Existing overlays should also be minimally impacted if you use aliases rather than explicit paths to nodes as they will transfer over. There may be a slight change around CSI and DSI as each connector now has a dedicated i2c bus, but GPIO, I2S and PWM will likely remain the same.
Backwards compatibility has been maintained as much as possible and most overlays should work unmodified.
