You could possibly do a quick switch if you had a helper and some giant towels. The person could have some type of rubber pad (like the kind you put a hot plate on) to slap over the hole when you pull it out, and if the bulkhead had a temporary fitting in it to act like a plug, you could push it into place and let the water pressure hold it as you screwed the nut on.
Draining the tank for 10 minutes is another option.
A friend of mine decided to replace a bulkhead under his massive reef years ago. His name was Travis, and the plumbing was in the way for his new skimmer. He got under the tank naked with a face mask and removed that bulkhead, water shooting everywhere and hitting the nearby oultlets, resulting in some fireworks. He got the new one in place and tightened and stopped the shower of water and sparks. It was a hilarious story, and we called him Naked Travis after that one. When he installed the skimmer after that, it turned out it would have fit just fine with the older plumbing. LOL Why did he opt to be naked? He knew he was going to get wet.
