I created a simple project with two screens and buttons on each to swap screens and then exported the UI code. It looks fine each button event to swap screens calls the function e.g.
You can find _ui_screen_change function in ui/ui_helpers.c but it’s intended to be an internal function of the UI folder I think (assumtion based on to the ‘_’ underscore prefix). If the screen already exists (created along with its child-objects) it doesn’t do more than calling LVGL function lv_scr_load_anim(), which you can use directly anyway, maybe in a simpler way.