The switch up happens between 6:30-7:00 pm most nights, so if you stay til closing, you can usually see them. As a bonus, they usually get fed on exhibit. Keepers are trying to get home at the end of their shift, so the earlier they can switch the lions, the faster they can finish their duties and go home
Shifting animals before closing is good for tired keepers. I remember hearing when I was a kid that keepers used to have a hard time of it getting elephant, Patsy to come in at night. They would start early with her because they knew from time to time she would flat out refuse and they would be there most of the night trying to convince her to come in because if she wasnt going in the rest of the herd stayed out there with their matriarch. Some you can leave out others you cant and poor keepers are stuck with those until they just come in. Keepers are such devoted people.