If you lift an area of lawn, then stack the turf upside down and it will break down over a year to give you lovely homemade compost that can go back onto the bed.
If you are planning to convert an area of lawn, the turf also needs to be lifted. That is unless you are planning to go no dig or do a lasagna garden as those beds can be created directly on top of existing lawns or weeds. If you lift an area of lawn, then stack the turf upside down and it will break down over a year to give you lovely homemade compost that can go back onto the bed.