Why can I not change my Apple ID

Have you logged into your Apple ID to see if the address shows under Reachable At, as a Rescue email address, or as a Notification email address?

Apple ID - Manage  

Apple hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Notification email address or a Rescue e-mail address, then it can't be reused with another Apple ID for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again.  You will need to get another e-mail address to use as your Apple ID or you can try waiting 30 days to see if that works.

What happens when you try?

when I type new apple id it says it's not available

Are you following the instructions cited in link at “Related Article?”