Since I’ve left, my partner has been really nice. Does this mean they have changed?

They may have, but it is unlikely. Statistically this is the highest risk time in terms of abuse (when the relationship is about to end or has ended).

Part of the reason for this, is that they have lost control of you and the situation. A way of attempting to regain control can be by being really nice.

It is very common for people, where being nice is not effective, to become very nasty as a different way of trying to get control. People can swing between nice and nasty in this way in a matter of minutes, hours, days or weeks, depending on the individual. Being nice can be another way of being abusive and trying to make you do what they want.