A window's direction is only part of its light conditions, since nearby buildings, glass and seasonal changes affect the plants beside it. Observe the actual light before deciding which species belongs on a sill.
Observe the light through a day
A window's direction does not describe everything a plant receives. Trees, nearby buildings, curtains and the distance from the glass all change exposure. Look at where direct sun actually falls and how that pattern changes with the season.
Match a named plant to the space
Check the requirements of the species or cultivar you want to grow. A plant chosen for a bright sill may struggle farther inside the room, even when both positions face the same direction. Compare its growth after a move rather than assuming that every south-facing window suits every plant.
Change exposure gradually
Plants moved into stronger sun may need a period of adjustment. Watch the foliage and check the pot's drying rate, since more light can alter water use. Keep leaves away from unsuitable hot or cold surfaces. If stems stretch or leaves become damaged, review the actual conditions instead of adding fertilizer to compensate for a poor position.
Related reading: Growing Orchids at Home: Start With the Right Conditions.