I think the time to move out of the 5-point harness is if the shoulder straps are on their heighest setting but the shoulder strap is not at least level with or higher than the child's shoulder.
Apparently if the straps are lower than the shoulder it is not as safe. That is what I was told anyway. However, I suspect that you need to be a certain weight for the normal 3-point seatbelt to work properly so if your child is tall and skinny you might have problems. Better to stick with the 5 point in that case.
What about something like this which is supposed to last from 9mths to 11yrs