If you wrote all the code, you can change the licence. But if you have external contributors, everyone must agree to change it.
If someone forked it in github, or have a copy at home or in gitlab, even press the history button in github, they can continue using the old licence for the old code, and "fork" the project and modify it using the old licence.
You change the licencee only for the future code.
Beware that changing it to MIT means that tomorrow Amazon can launch a hosted version and earn millions and not send you a dime. I like MIT and I'm not worried about Amazon for my projects, but many people don't understand this side effect.
If you wrote all the code, you can change the licence. But if you have external contributors, everyone must agree to change it.
If someone forked it in github, or have a copy at home or in gitlab, even press the history button in github, they can continue using the old licence for the old code, and "fork" the project and modify it using the old licence.
You change the licencee only for the future code.
Beware that changing it to MIT means that tomorrow Amazon can launch a hosted version and earn millions and not send you a dime. I like MIT and I'm not worried about Amazon for my projects, but many people don't understand this side effect.