When you make your free tier smaller, what do you owe the people already using it?

I've just made my free tier worse for a group of people who did nothing wrong, and I can't work out what I owe them.

Background, anonymised as best I can. Until this week you could use the core function of the thing I build without an account at all — turn up, get a result, look at it. You only had to identify yourself if you wanted to keep the output. This week I put a free account in front of it. Same allowance, same features, no card, but you now sign up before you get anything.

The reasons were dull and real. I couldn't attribute anything to anyone. I couldn't tell a returning person from a new one, so I had no idea whether people were coming back. And an anonymous endpoint that costs me money on every call was an abuse problem I was going to have to solve eventually regardless.

What I didn't think through is what to say about it. Three things I'm stuck on:

• The people affected are, almost by definition, people I cannot contact. They never gave me an email. That was the entire point of the thing I removed.

• The change makes the product better for anyone who signs up and worse for anyone who won't, and I don't know how to weigh those against each other when the second group is invisible in my analytics by construction.

• Every draft announcement I've written reads either defensive or like I'm dressing it up as a feature. "We've added accounts" is not what happened.

I also can't tell whether this matters or whether I'm agonising over something maybe forty people will notice. I don't have the numbers to know — which is itself a consequence of the thing I just took away.

So for anyone who's tightened a free tier: did you announce it at all, and did it cost you anything measurable? And is there a way of saying "this is now worse for you" that doesn't read as spin?