l_m,

The 3/4" will decrease loss of pressure by providing a larger volume with the new pipe. It will give you more consistent pressure across the entire lateral by giving more available pressure from the increased volume that the system can use before being restricted. This restriction is caused by the cumulative pressure loss from each orifice ( too many holes). I don't know how the orifices became fat and leaky (chewing insects/rats/squirrels etc?) but the larger pipe would help with the pressure loss with this problem as well. Your flow is dictated by the number of holes and their diameter for your particular system pressure. So a larger pipe diameter does not necessarily mean more flow in the pipe. Hope this makes sense Aloha