The Union's two-navy shipbuilding problem

Could the Union counter HMS Warrior by shifting from monitors to seagoing ironclads?

Yes, but it would have to build an ocean fleet in addition to river, coastal, lake, blockade, and army requirements, while Britain could concentrate on blue-water warships.

War with Britain would push the United States toward ocean-going ironclads rather than reliance on monitors alone. Those ships would have to be built alongside river armour, coastal monitors, Great Lakes forces, and the fleet enforcing the Confederate blockade. The same shipyards and imported materials were also competing with the demands of a massive land war. Britain could concentrate naval construction on successive blue-water ironclads while limiting its army commitment largely to Canada. American industry could eventually respond, but it would have to create several urgent force structures at once.