Trying to change my apple id to one of my email address but I'm getting error - "this email address is not available. choose a different address"

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 Reachable at or 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. There have also been posts that if you delete an ID associated with the address, you can never reuse the address. 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.

You are welcome. You did note the part about there have been some posts that stated you can never reuse the address.