Booking new LL during grace period?

One of the 3 ways to book a new LL is to book when the return window for your most recent LL expires. What happens if it expires but you still want to ride during the 20+ minute grace period? Would booking a new LL negate the current one of would you be able to book a new one and ride the one you’re currently headed to?

I hope this question makes sense Thank you so much for all the videos. It helped so much during my last trip and anytime someone new is heading to Disney, I make sure to tell them to check out your channel.