Other Q&As on RPG.SE have discussed the by-now-familiar rules for mounted combat, including the differences between controlled and independent mounts. See, e.g., When can you choose to control a mount? Mounts summoned via find steed, however, are a special case. Per this 2018 guidance from Jeremy Crawford,
when you ride the mount [summoned via find steed] in combat, you decide whether it follows the rules for a controlled or an independent mount.
What remains unclear, however, is precisely when a caster who is already mounted on the summoned steed may, or must, make that choice. For example, it could be:
- When the DM calls for initiative rolls.
- Only on the caster's first turn.
- Only the mount's first turn.
- On any turn the caster takes.
- On any turn the mount takes.
- On any turn whatsoever.
Some of these options would require the caster to choose only once and stick with it. Others would conceivably let the caster change her mind each round, or even multiple times each round. Which (if any) is the correct way to handle mount control using find steed?
Any reasoned and supported answer is acceptable. If, e.g., Crawford's guidance is the only applicable source, then an answer well-reasoned from that -- especially one with experiential support -- is acceptable.