Hey Charles, Jesse with the Land F/X irrigation team here. When you're mimicking pipe data, you're not only selecting the pipe classes you want to mimic from and to, but also the sizes. So if I were to mimic 1" PVC Sch 40 to 1" PVC Sch 80, I am telling the system that I only want to swap 1" Sch 80 for 1" Sch 40. This will leave all other placed sizes of the PVC Sch 80 unchanged.
Would you like to share more about what you're trying to achieve? We could have a better workflow for you.