St John's Point lighthouse at Killough dates back to 1839.  It was used as a marker on a test run for the Titanic.
The building is now maintained by the Commissioners of Irish Lights, which plans to replace the sweeping beam with a static light-emitting diode (LED).
However, conservationists have begun a campaign to keep the existing light on.
The campaigners have enlisted the support of the South Down MP, Margaret Ritchie, who has written to both British and Irish governments, asking them to intervene to save the traditional lamp.
It follows a "heated" meeting at the weekend, when campaigners invited representatives of the commission to County Down to witness the strength of local opposition to the planned upgrade.
Eileen Peters, from Killough Community Association, told the Down Recorder that the lighthouse beam is of "cultural and historical significance" for the village and the surrounding area.
"There are those who love to see the lighthouse beam shining out across the bay and it's our understanding that the LED light will not stretch as far out across the water and that rather than rotate, will flash on and off," Ms Peters told the paper.
The Commissioners of Irish Lights is a maritime safety organisation based in the Republic of Ireland but which has responsibility for lighthouses and other coastal infrastructure throughout the island of Ireland, north and south.
The commission's director of operations and navigation services, Capt Robert McCabe, said the planned LED replacement would improve safety and reduce maintenance costs at the site.
Capt McCabe travelled to County Down for the weekend meeting and said he acknowledged the concerns of the campaigners and would consider their views.
He said the commission was "fully committed to maximising the potential of our heritage and history at St Johns Point" but he told the BBC that the commission's top priority was to provide the "best aid to marine navigation".
He added that it was important to remove mercury from the lighthouse, because of the risk that exposure to the element poses to human health.
The existing lamp has a Fresnel lens - an early 19th century invention that projects a long-distance beam using a modest light source.
The huge lens is the height of two average living rooms and weighs six tonnes.  It rotates inside a mercury bath.
In a statement, Capt McCabe said the lens would not be removed from the lighthouse, but the replacement of "obsolete" filament lamps with an energy efficient LED light source would provide a "more reliable and better quality light".
"Environmentally it is also a far superior solution allowing the removal of mercury and gas oil from the station. Mercury, being a hazardous and poisonous substance, is being removed for health and safety reasons."
However, he campaigners said there has never been an incident of mercury poisoning at the site and argue that many lighthouses in parts of England have safely maintained their traditional lamps.
The area's MP said the Commissioners of Irish Lights were accountable to both the UK and Irish governments, due to their cross-border remit.
Therefore, Ms Ritchie has written to Prime Minister David Cameron, Irish Prime Minister (Taoiseach) Enda Kenny and the UK and Irish transport ministers this week, asking them to step in to preserve the existing light.
Sinn FÃ©in MLA Chris Hazzard, who helped to organise the campaign meeting in the nearby village of Ardglass last Saturday, said the residents' views must be taken into account.
He said many older members of the coastal community found the traditional sweeping beam "soothing" and reassuring, describing it as a source of "solace" in stormy weather.
Mr Hazzard said the commissioners removed the fog horn from St John's Point four years ago "without consultation" and the residents did not want the same fate to befall the traditional beam.
Capt McCabe agreed that heritage campaigners were not consulted about the fog horn, but said they did consult mariners, and added that fog horns were removed from all of their lighthouses in Ireland at the time.
The commissioner said the exiting light at St John's Point has a nominal visibility range of 24 miles, while the LED light has a shorter range of 18 miles, but said this was still well within safety limits.
He said the new light would be brighter and during power failures it could be backed up by a battery supply rather than a generator, cutting down on cost.
However, he said they were keen to try to find a solution that would meet the requests of both heritage campaigners and the requirement to modernise the lighthouse with the very latest navigation equipment.
Capt McCabe said two lighthouse keepers' cottages at St John's Point site were currently being renovated and added that the whole site has the potential to become a "top class tourist attraction" based around lighthouse history.
He added that commissioners would consider the campaigners' views carefully before applying for listed building consent to make changes to the lighthouse.
The lighthouse, which now stands about 130ft (40m) in height, has undergone many renovations since the Marquis of Downshire laid its foundation stone in 1839.
In 1875, the main light source was converted from oil to coal gas.  By 1908 it was powered by vaporised paraffin and it was eventually converted to electric in 1981.
1981 was also the year that live-in lighthouse keepers were withdrawn and the building became automated, with a part-time attendant taking charge of the station.
A storm is brewing in a small County Down coastal community over a plan to replace a traditional lighthouse beam with a new, cheaper lighting system.