Arial Black
Adventurer
But if I say that the spell works slightly differently in my campaign then it does.
Bearing in mind that one of the DM's jobs is to be fair, then it is wise to be seen to be fair.
If, at the start of the campaign, or as a player was about to choose banishment as one of his spells known, you told the player that you rule that the spell works differently in your campaign and this is how it works, then the player can choose to take the spell anyway or choose a different spell instead, and if he comes across another caster he knows what to do.
But if you said nothing about how you change this spell and then during a crucial combat change the way the spell works such that the player's solid 'win' turns into a 'lose'....
...it looks very much like you are cheating.
This is to be avoided.