Hi bart 
This should work. Obviously, do a backup and carefully look for any unwanted results in the previous masonry posts… 
posts = Post.where('raw LIKE ?', '%<div data-masonry-gallery>%')
posts.each do |p|
p.update!(raw: p.raw.gsub(/<div data-masonry-gallery>(.*?)<\/div>/m, '[grid]\1[/grid]'))
p.rebake!
end