I'm on a five-line family plan, and retention doesn't usually do a lot of favors for people in that case. That's been my personal experience and that of a friend.
If they are all expiring around the same time, you will have leverage. The problem with multiple lines is keeping them all synchronized. But the key with retention is to call and if you don't like the answer call again and repeat until you get the answer you want.
Golden Rule of a Deal: If an item has been offered validly at a price of $X at any time, then never pay more than that in the future. It has been discontinued or will appear again at that price soon enough.