Yes, the key difference between ego states and the alters of DID is that the alters can take control and switch. With ego states, which everyone has, the ego states never take control. Here are the first and second features (of four) of the DID diagnosis (from the DSM). Both of these are mandatory to have DID.

A. The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self.)

B. At least two of these identities or personality states recurrently take control of the person's behavior.

When B happens, the other alters often do not have memory of this. The alter who takes control is the one who has the experience. The other alters "lose time."

With ego states, you may have "A" (although not necessarily), but you do not have "B". That is how it is for me: I have A but not B, which is very common. So I am not DID according to the DSM.

It has best been explained to me that there is a continuum of dissociation with DID at one end of the spectrum. Maybe at the other would be slight dissociative behaviors such as "zoning out." In the middle are ego states.

I have also read that many people cannot access their ego states without hypnosis. But I have, so I know this is not a universal.

Ego states can also be very slight organizing principles, not necessarily fully developed personalities, although they can be that too. For example, my T told me one might have an ego state of "how I experience a sunset." Other ego states might be organized around "how I function at work." Or "how I make a killing in the stock market every day." Or "the little girl who was abused when she was 3 years old." Or "the teen who lost her father in a horrible car accident." There's a whole range.

I think the key is to realize there is a continuum and that we all fall along it somewhere.

I hope that is helpful!