Hey Braxbro,
I looked into it and I don't think it's possible to convert existing saves safely, as the commands I use to transform the starting area appears to destroy all entities and players in the transformed zone while it's processing.
Theoretically if you had any landfill outside your current base to stand on you could pick up everything and run the following command to convert the starting zone into landfill:
/c
    local surface = game.get_surface("nauvis")
    local tiles = {}
    local tileIndex = 0
    local island_size = 25
    local minDist = -island_size + 1
    local maxDist = island_size
    for x = minDist, maxDist do
      for y = minDist, maxDist do
          tiles[tileIndex] = {name = 'water', position = {x, y}}
          tileIndex = tileIndex + 1
       end
     end
    surface.set_tiles(tiles)
    tiles = {}
    tileIndex = 0
    for x = minDist, maxDist do
      for y = minDist, maxDist do
        tiles[tileIndex] = {name = 'landfill', position = {x, y}}
         tileIndex = tileIndex + 1
       end
     end
    surface.set_tiles(tiles)
For starting space:
I'm not understanding the distinction between 'starting space' and 'pre-research space' . 
If you'd like to be able to set the size of the starting island in the mod settings I can add that, but I'm pretty sure I can't make the map preview correctly resemble the starting island due to quirks of when the map generation functions get created vs when the mod settings are available.