De pie and en pie are interchangeable in some contexts, including 'ponerse en/de pie'. However, estar and seguir en pie are preferred when used to mean 'standing' figuratively, while estar and seguir de pie are preferred when referring to a person's posture, i.e. standing on their feet.
-El público se puso de/en pie — The audience stood up
→Puesto en pie (de pie), el público lo aplaudió — The audience gave him a standing ovation [Se prefiere 'en pie' tras el participio]
-¡En pie (de pie), todo el mundo! — Stand up/ On your feet, everyone! [En imperativo, sin verbo, se prefiere 'en pie']
-El hombre estaba de pie junto a la ventana — The man was standing next to the window
-Yo seguía de pie en medio del salón — I was still standing in the middle of the living room
-A las ocho ya estaba en pie — At 8 o'clock he was already up (and about) [≈Levantado]
-La oferta sigue en pie — The offer still stands
-Ha sido un día duro, pero ya ves, sigo en pie — It's been a hard day, but I'm still standing/on my feet, as you can see [≈Levantado; no se refiere a su postura, sino a su estado]
-El edificio original sigue en pie — The original building still stands
