Light spectrum isn't as important but i have noticed that my autos do like the MH early in veg. They green up much better than with HPS but generally you are correct. There are a lot of people that only use HPS.
As for the light switch, you could do it any time you are ready. I would consider dropping it an hour a week, like 19/5 the first week and 18/6 the second. Less stressful that way. There really is no reason to go to 18/6 unless it is strictly a cost savings goal. They will flower just fine on 20/4 and production will probably be a little better all things considered but it is entirely up to you as well. They will do quite well under 18/6 also.