A multi-million pound revamp of the award-winning extension to the National Museum of Scotland is set to be carried out to coincide with its 25th anniversary, Scotland on Sunday can reveal.Plans for the overhaul are expected to be well under way by 2019 - once long- term refurbishment work on the Victorian-era museum on Chambers Street in Edinburgh is completed.The striking landmark, designed by London-based architects Benson & Forsyth, and initially known as the Museum of Scotland when it was opened by the Queen in 1998, is dedicated to telling the story of the nation.Home to many of Scotland's treasures, the gallery spaces trace the country's development back to its origins, exploring the evolution of its landscape and the impact of 10,000 years of human existence.The new complex - which filled a gap-site on the corner of George IV Bridge and Chambers Street - made the shortlist for the Stirling Prize for Britain's best building the year after it was unveiled. It was named one of the 20 best modern buildings in Scotland by the architectural magazine Prospect in 2005.Despite pre-dating the opening of the Scottish Parliament, in 1999, just one of its major displays has been altered since then, although there was controversy over several changes made to the main entrance hall in 2004.Last week it emerged that work on the final two phases of the ongoing GBP70 million plan was expected to start in the autumn and could be completed by 2018 or 2019.By then, National Museums Scotland is expected to be exploring options for the Scottish Galleries, which take up the entirety of the new extension. It is understood that this further revamp would be ready well before 2023.Gordon Rintoul, director of the museum, said 25 years was the normal "lifespan" for gallery displays in major attractions.He said: "We haven't set a timeframe for the Scottish Galleries yet. Our real focus at the moment is in delivering all the elements of the masterplan for the old building here in the timeframe we set out in 2004. We've always viewed the current masterplan as something that could take up 15 years."He added: "We will clearly be starting to look at the Scottish Galleries at some point in the future. They were opened in 1998, which is 16 years ago now, and they were designed to have a 25-year lifespan It is normal with museums like this which, after all, is the largest outside London, that you're essentially in a process of continual renewal."I think [the Scottish Galleries] are particularly popular with people visiting Scotland who are looking to get an overview of the nation and its history, but things move on."The overhaul is likely to transform existing exhibition spaces, with hundreds of new exhibits going on show for the first time, and use the latest technology to create more "hands-on" displays.Rintoul also threw his support behind plans to create a new cultural quarter behind the museum, weeks after the Filmhouse cinema launched a consultation on a possible move to the site, which is jointly owned by Edinburgh University and the city council.