Sometimes switching foodplant is needed because of some problems related to a specifical plant. For example, as you could see in the discussion "samias died", I told you that I had some troubles with Prunus laurocerasus so I switched to Ligustrum. Some breeders state that it is better to switch during the L3, when switching foodplant is not too difficult. Also if you notice that you don't have enough food to rear larvae on the same plant, you could try to switch to another known foodplant of the species.