change user ID on iphone and not lose current apps

I am not clear as to what you mean exactly.

Here's how you change the email address (Apple ID) you use for your Apple Account while retaining everything in your account:

If you have an iPhone or iPad:

1. Go to Settings, then tap your name.

2. Tap Sign In & Security.

3. Next to Email & Phone Numbers, tap Edit.

4. Tap the Remove button next to the email address, then tap Delete.

5. Tap Choose Another Email Address and enter your device passcode if asked.

6. Enter the new email address, then tap Continue.

7. Enter the verification code that was sent to your new email address. Read this support article to learn what to do if you didn’t get the email —> If you didn't receive your verification or reset email - Apple Support

8. If you encounter issues with certain services after doing this, this outdated support article may still have useful suggestions --> What to do after you change your Apple ID or password - Apple Support

Refer to the support article Change your Apple ID - Apple Support :

- If you don’t have an iPhone or iPad the support article says to, “go to https://appleid.apple.com and sign in. Then follow the onscreen instructions to change the email address associated with your Apple ID.”

- For general information, and other issues such as changes or addresses not being accepted.

- Note a recent change to the article where you can now change the email address without having to have access to your old email account.

If you mean use a different Apple account on a phone previously used by another person, you are much better off completely resetting it, otherwise you will run into various issues because of residual settings from the previous user. See What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support