Ask the designer
It is always a good idea to consult your lighting supplier for details as lighting can come at different colors and intensity. It is a good rule of thumb to space the down lights between 1.5 meters to 2 meters to ensure the room is evenly lit.
It is possible to retain the existing roof depending on the scale of work being done which may save on cost. However it is best to consult your builder/ structural engineer about the feasibility of retaining the existing roof. Also consult them about both options before making and big decision especially in relation to structural issues.