##The PHB supports what you did.
Ultimately, the DM's authority extends even to this.
Ultimately, the Dungeon Master is the authority on the campaign and its setting, even if the setting is a published world. (PHB, p 6)
But it's still best to discuss it with your play group
"Because I said so" does not always sit well with a given group of players. Whenever you retcon something in a game -- not just in D&D -- I have found that it is best to (1) discuss it with the players, (2) explain why, and (3) and get their buy in. If they don't buy into it, you've got to consider "how important is this retcon to our continued campaign?" It is best to arrive at an agreement, or "get on the same page" before you start the next session.
Sometimes, we DMs and GMs fess up and tell the group: "eh, I think I made a boo boo there, here's how it works from this point forth. The gods were crazy that day."
Or something like that.
- We ran into that in our group last Monday night. I allowed a player to cast sanctuary and firebolt in the wrong order (he should have gone firebolt/sanctuary to avoid sanctuary dropping from the attack) but since he's new to the class, and I knew what he was trying to do, and it was good for the party for him to do this, I said after the battle "Yeah, it worked out this time ... but remember, going forward, it is supposed to work like this {explained the mechanics}." We are on the same page.
Notes:
- retcon is a term often used to describe what you did with the retroactive change.
- You may find this question and answer "Is the DM always right?" useful when dealing with some situations as the DM.
On a humorous note: if the DM's wife disagrees with the DM, can the DM ever be right? grin