Violinist Nicola Benedetti and the city's planned new concert hall off St Andrew Square, which Ms Grigor's Dunard Trust is also backing, are now official partners of a vision to breathe new life into the 192 year-old landmark on Calton Hill.
A charitable trust that has already secured planning permission to create a new home for St Mary’s Music School and a concert venue at the site has drawn up more ambitious plans following the rejection of rivals’ plans to turn the A-listed building into a luxury hotel.
However, the changes have seen the cost of the project soar by £10m in the space of nine months.
Ms Grigor’s trust has pledged £45m to pay for all restoration and redevelopment works at the site, as well as another £10m to create an endowment fund for the centre’s future maintenance and running costs.
Part of its grounds will be transformed into a new public garden, while the original western pavilion at the site will become home to a visitor centre, gallery and cafe.
Public events at the new National Centre for Music are expected to be jointly programmed with the nearby Dunard Centre, which Ms Grigor has previously pledged £35m to.
Ms Benedetti’s own foundation, which puts on orchestra-based workshops for young people and teachers, is expected to play a key role in the running of the National Centre for Music, under a vision for it to be “a catalyst for Scotland to fulfil its potential as a world leader in classical music education, creating an entirely new way for the nation to engage in every form of music”.
Other official partners backing the new centre include Traditional Arts and Culture Scotland, the Nevis Ensemble, Drake Music Scotland, Pianodrome, the Dunedin Consort, Live Music Now Scotland and Fèisean nan Gàidheal.
The proposed new home for St Mary’s Music School, which is seeking to “acquire” the building so it can relocate from the West End, had been championed by the city’s main heritage bodies.
The former Royal High School, which was previously proposed for a new national photography centre and home for the Scottish Parliament, has been lying largely empty since 1968.
Ms Benedetti said: “The National Centre for Music presents us with an unprecedented opportunity to enrich the cultural life of Scotland and to serve as a beacon of true 21st-century music education for the world to see.
“Thanks to the generosity and vision of Carol and the Dunard Fund, we have the means, as well as the collective will and dedication from all walks of Scottish life, to realise a revolutionary vision.
“The National Centre for Music will be a warm and welcoming place for all ages, abilities and backgrounds, where people can come together and be uplifted through participation in and appreciation of music."
Joanna Baker, executive director of Impact Scotland, the charitable trust pursuing plans for the £75m Dunard Centre concert hall, said: "The National Centre for Music is genuinely world-class in ambition, excellence and access and allows Edinburgh to continue to assert itself on the world stage.
“We’re particularly excited that new ways of collaborating with St Mary's Music School educationalists at the former Royal High School building also opens out very interesting possibilities and links with the Dunard Centre's partners".
The biggest changes to the original building, designed by architect Thomas Hamilton, will be to create a new main entrance and foyer to open up access to its main auditorium, which will have a capacity of around 280 for concerts. Two other spaces for public performances will be created.
The city council invited bids for the former Royal High School in July, more than a decade after announcing it had agreed to a deal to allow the site to be turned into a hotel. However, despite the global chain Rosewood agreeing to operate the hotel, councillors twice rejected plans for the project over fears that planned extensions would harm long-standing views of the historic site.
The Royal High School Preservation Trust, which has been pursuing plans with the music school for the past six years, has made a normal offer of £1.5m to the council.
It claims the project will be worth nearly £100m to Edinburgh's economy in its first 30 years.
Trust chair William Gray Muir said: “We’re effectively taking the whole site site as far back as possible to Thomas Hamilton’s original conception and how it sat on Calton Hill.
We want to ensure that views of the site are returned to as great a degree as we can to what they would have been when the Royal High School was first built.
"This involves removing later additions on the east of the site, as recently as the 1950s, and replaces them with lower buildings.
“To the west, we want to consult about having a relatively small new entrance pavilion, in response to some criticism of our previous proposals that two new towers to the rear of the building would obscure part of its north facade from Calton Hill.”
Richard Murphy, the award-winning Edinburgh architect who has been working on the project since its inception, said: "This setting has been compromised for about 150 years and our proposed new buildings are designed to be seen as a low-lying terraced landscape.
"Alterations to the main building are restricted to a new entrance, invisible from the street, but otherwise new work is located in areas of the building which have already been significantly altered.
"A key part of our proposal addresses the curiosities of Hamilton’s very under-used external staircases, now revived as a major new entrance route for the public performance spaces.”