Overworld sprites should "just work", draw_sprite_gridbased uses a percentage of the sprite asset's size so the only thing that matters is that it's a 4x4 grid.
For the tiles, change the TILESIZE macro-constant to 32 (it's set in init_constants) and then also update the settings of tileset_indoors and tileset_outdoors to be 32 (the underlying sprites also need to be resized to 640x512 for the information about how many columns of tiles are in the sets to be accurate.
Changing the sizes should be safe (I use the TILESIZE constant everywhere) except I don't know what Game Maker will do with pre-existing tile layers when you change the tilesize. It's probably best to make a backup of your project (File --> Export Project --> YYZ) before you do this.