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Obviously the safety devices are very different. Though of the same family of products. The most notable difference would be efficiency. A high pressure boiler is built of much heavier material with much higher BTU's in mind. The thermal efficiency is based on it moving vast amounts of super-heated water through the system, and maintaning that temp. A low pressure boiler uses much thinner material and low BTU's because the thermal demand isn't by design nearly as high.
Well first off, there will be spring loaded valves to be removed, these are in place to ensure proper heating of the high temp system, and to keep the boiler from flash vaporizing water in the tubing and allowing a dry-fire situation. Condensation return systems will become redundant. outlet sizes are normally larger in a low pressure system because fluid is being moved rather than steam. Circulation pumps and lower temp sensors in the system will need installed. Normally the fin-tube systems in each room will have to be considered based on size. It takes a larger unit to produce the same amount of heat in a low pressure system. Fuel delivery will have to be considered. the lower pressure system will not need high pressure natural gas, so an in-line regulator matching the BTU demand will need to be installed.
the flue gasses will not be as hot, so a smaller vent is needed to prevent back drafting of the flue system
Obviously this is not an all-encompassing list. I can't inspect your situation and tell you absolutely everything, but these are the basics.