I will post a long section about things to try when you have forgotten security questions but the bottom line is if you don't remember the security question answers then it may be very difficult to resolve the issue of the old ID. If you really don't care about the old account and its contents then you may find it easier to ask Apple to reset the iPhone. Do this online here --> https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what constitutes proof. Prior to this new online service Apple typically required the original receipt issued by an Apple Authorized dealer and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).
If you want to try to reset the security questions to be able to reset the password:
"If you forgot the answers to your Apple ID security questions" click here --> https://support.apple.com/HT201485
If you see the message "not sufficient information" read this link:
Security and your Apple ID - https://support.apple.com/HT201303
"Without proof of your identity ... and other carefully selected criteria, Apple Support can't help you perform any actions on your account."
Apple does have "Account Recovery when you can’t reset your Apple ID password" ( https://support.apple.com/HT204921 ) which it specifies is available for "when you don’t have enough information to reset your password." However, it also states it is "if you use two-factor authentication" which you may not be if you are still using security questions. You can investigate if this is still available to you (please report back if it is so I know this is a viable option for others in this situation).
It may ultimately prove very difficult to find satisfactory resolution to forgotten security questions if you get the "not sufficient information" message (e.g., https://discussions.apple.com/thread/252410816 ). Apple is very strongly urging users to use Two Factor Authentication instead of security questions. --> https://support.apple.com/HT204915 "With two-factor authentication, you don't need to remember any security questions. We verify your identity exclusively using your password and verification codes sent to your trusted devices and phone numbers. When you enroll in two-factor authentication, we keep your old security questions on file for two weeks in case you need to return your account to its previous security settings. After that, they're deleted."
With that in consideration you can try to "Contact Apple for help with Apple ID account security". Click here --> https://support.apple.com/HT204169 If the online option does not resolve the problem I suggest trying to get a personalized response with a chat or telephone call. To talk by telephone: Contact Apple for support and service - Apple Support --> https://support.apple.com/HT201232
For other support options including those if your country does not have telephone support: https://support.apple.com/choose-country-region To chat, choose any topic, (not Apple ID) continue working your way down options until you find one with chat support, then ask to be redirected.
(Note: If your country is not listed, there may be a support option at the end of each region section for "other".)
