I am not sure what your lazerette or tank area look like but if you are concerned about the integrity of the boat then support everything that is being supported by the plywood then cut out the old section, repair or install new tank and put back the plywood etc. One thought if you are not removing the entire piece of plywood then you may not need to worry about the integrity since you will still have some support. If you are not going to move the boat etc. during tank extraction what could be the harm? Consider all that is being supported then support as necessary or consider leaving as much plywood as possible. If its the floor to the cockpit it my not matter that much just cut out what you need to remove the tank. I would seriously consider removing the old tank and repair. In the future you might be glad you did since you could have a headache trying to secure an old tank on new. Besides do it right the first time even if it takes more time vs. a quick fix. As posted above, pictures would be helpful to see your problem.