Can i use new email or apple id to an existing phone

You can but if those apps that stay on the phone need updating, they will need the password for the Apple ID that was used to download them. You simply sign of of the Apple ID and it will ask do you want to save iCloud data and you checkmark what you want to save like contacts, photos etc. Know that signing out of the Apple ID does not delete the apps. Only an erase would do that. So when you sign out of the Apple ID and save what you want in the iCloud data like contacts, etc…the apps will stay no matter what you choose. Also, if you are signed into any third party email accounts like Gmail etc, they also will stay. With photos, you would want to make sure you turned off iCloud Photos and download to the device is selected. Make sure the device has enough hard drive storage to store the downloaded photos. (Settings-General-iPhone storage)If there are a lot of photos, give enough time to let them download to the device before completely signing out. Example, if you have thousands of photos, I would wait overnight. You can keep checking the photo app at the bottom to see if the photos have downloaded. Remember when you sign in/create a new Apple ID on the phone, it will ask what to do with the data on that phone that you saved….choose MERGE