Hi
The churchyard itself describe by London Gardens online (link in the London burial guide on Rootschat London and Middlesex boards) is a public garden (Garden museum) like many churchyards.
http://www.londongardensonline.org.uk/index.html 'the former churchyard behind the church laid out as a C17th style knot garden........A wild garden was created in 2007 in the former churchyard in front of St Mary's.'
In 1895 the churchyard was described as three quarters of an acre and that it was being considered to lay it out (meaning remove the gravestones, usually to the side of the church) and turned it into a public garden.
St Mary's Lambeth had an additional burial ground in the High Street also called Paradise Row burial ground which was one and a half acres in 1895. It was 'laid out' in 1884. There does appear to be green space (Google map) at the junction of Old Paradise Street and Lambeth High Street.
The London Burial Grounds website (link in guide) describes it as a pleasant public park.
http://www.burial.magic-nation.co.uk/bglambeth.htmSince both the churchyard and the additional burial ground still exist you would have expected during the conversion to gardens work was by necessity done, but it wouldn't necessarily involve reinternments anywhere but in the two spaces themselves.
Because of the pressure on London churchyards they were far from pleasant places in the first half of the C19th before they were closed to further burials in the early 1850s.
London burial grounds describes the additional burial ground
'At a short distance from the church is another burying ground, belonging to the parish; it is divided into the upper, middle, and lower grounds. It is very much crowded, and the tomb-stones are deeply sunk in the earth; the state of the ground has rendered it necessary to discontinue the practice of interment. Bones are scattered about, and a part of the ground has been raised. The neighbourhood is thickly populated; the soil is very moist, and water flows in at the depth of four feet.'
The estimate for how many people were buried in the small churchyard around the church is 26,000. To get so many bodies into the churchyards charnel houses would be in use.
When the church crypt was cleared that would require a mass reburial in one of the large civic cemeteries.
Since the church is closed the archive that may know this information would probably be the local archives in Lambeth as they hold the church vestry minutes.
http://www.lambeth.gov.uk/Services/LeisureCulture/LocalHistory/Archives.htmRegards
Valda