If it's a historical story, climbing over a wall might also mean having an affair. Forgot the exact phrase for this tho.
in computer slang it means overcoming or circumnavigating the Great Firewall of China. in terms of entertainment fandoms it usually means changing the idol or group that you stan. I've also seen it used in the same manner to describe someone changing loyalty from on country to another in a webnovel. in general, it can mean overcoming a limit, hardship, or physical/psychological barrier.
well the title was: (I'm a fan of both so I'm saying it first, I'm not dissing any of them) "Xiao Zhan fans climbed Cheng Yi's wall"