I got my pac number 3-4 weeks ago, and I stressed to the BT lady I emphatically do not want my contract to extend beyond today. She said something along the lines that it's no problem.
Also, it's not easy swapping sim cards. I have to bend a paperclip and poke in this hole!
Your BT sim will work until the day it moves to Lebara, so if it was me i'd use the BT sim over the weekend to save giving out the Lebara number for the sake of 2 days. But yes maybe could have been explained better, as there's no way to avoid overlap.