Are the instructions given in the "Planar Binding" spell part of the casting time, or can instructions be amended during the spell duration?

Question

The Planar Binding spell is a 5th level abjuration with verbal, somatic and material components and a casting time of 1 hour that allows the target a Charisma saving throw. If the target fails the save, it "must follow your instructions to the best of its ability".

Are the instructions specified as part of casting the spell? Or are the instructions given after the spell has finished casting? Can instructions be updated/amended throughout the duration of the spell, or is there a one-time period during which the instructions must be fully specified?

Although the duration for the 5th level version of this spell is 24 hours, the duration increases as higher spell slots are used, lasting a year plus a day when a 9th level spell slot is used. I mention this because the longer the spell lasts, the more important the answer to this question becomes.

Answer 1

It means it follows any verbal orders you give it during the spell duration

There is no ambiguity in the spells description. A creature affected by the spell

is bound to serve you for the Duration

The mention of instructions without a note for timing is simply the english meaning of the term. You tell it to do something, it is forced to do so (or at least a plausible literal interpretation of what you said). There's no limit on when or how often you can give it instructions, because the spells effect isn't to allow you to give it instructions, it is to force the creature to be your servant for the duration.

This is also why this line exists:

If the creature carries out your instructions completely before the spell ends, it travels to you to report this fact if you are on the same plane of existence.

I.e. if you have given the creature an instruction which it can complete, it must report the completion to you – so you can give it more instructions if you wish. Otherwise you’d have to end every single command with some variation of “and when you’re done, return to me at once and await further instructions”.

Answer 2

The Sage Advice Compendium has the following question/answer:

Is the sentence of suggestion in the suggestion spell the verbal component, or is the verbal component separate? Verbal components are mystic words (PH, 203), not normal speech. The spell’s suggestion is an intelligible utterance that is separate from the verbal component. The command spell is the simplest example of this principle. The utterance of the verbal component is separate from, and precedes, any verbal utterance that would bring about the spell’s effect.

By analogy, the 1 hour casting time of Planar Binding is filled with mystic words, and the instructions are intelligible utterances that are separate from the verbal component.

If the instructions were part of the casting time, it would answer the second part of the question (no, it would not be possible to amend the instructions). But since the instructions are not part of the casting time, there is no explicit guidance about whether they can be amended and thus it is up to the DM to rule on this.

Personal opinion: From a game-balance perspective, allowing the caster to continually update the instructions makes an already inordinately powerful spell that much more powerful. As a player, I would prefer that freedom, but as a DM, I lean toward limiting the power of the spell. Focusing on RAF (rules as fun), there are more opportunities for interesting interactions between caster and target if the instructions must be enumerated at the completion of the casting time (for example, in the 10 minutes after the spell is cast and the target fails their saving throw, all instructions must be specified and are locked in place for the duration of the spell).