Previous Apple account? Therein lies your problem…
You clearly have more than one AppleID account - and your old iPad is associated with your old account. As such, you must use the account credentials (username and Password) for that account with your old iPad.
An AppleID account is intended to be used for life. If you need to change the primary email address that identifies your account, or your secondary (recovery) email addresses or trusted telephone numbers, you simply need to update your AppleID account as necessary.
Change your Apple ID - Apple Support
Change your Apple ID password - Apple Support
What to do after you change your Apple ID or password - Apple Support
Change, add, or remove Apple ID payment methods - Apple Support
Few of us will have the luxury of forever keeping the same email addresses or telephone numbers - sometimes these needing to be changed for a variety of reasons. However, updating our AppleID account, maintaining access to our data and purchases, is easy and relatively straightforward - accommodating life’s many changes as they occur. Using a completely different AppleID account can introduce many complications and issues, not limited to losing access to paid Apps, subscriptions and services - these being forever tied to the AppleID account from which they were purchased - and older “unused” devices that will be associated with an entirely different AppleID.
If you do wish to change and use an entirely different AppleID, first ensure that you have disabled the Activation Lock that will otherwise cause difficulties in the future. You can disable the Activation Lock by disabling the Find My service from iPad/iPhone settings before trying to use a different AppleID account:
Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF
Having disabled the Activation lock, you can now sign-out from your old AppleID:
Settings > [Your Name / AppleID] > Sign Out
If you don’t have access to the old AppleID account credentials, you will need to recover access to the account. If you have forgotten your AppleID or associated password, these can be recovered here:
https://iforgot.apple.com/
To recover your credentials, you’ll need access to any one of the following:
- Your primary email address mailbox that corresponds with your AppleID
- Any of the secondary/recovery email addresses that should be configured for your AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID
More information about recovery of your AppleID and associated Password:
If you forgot your Apple ID - Apple Support
If you forgot your Apple ID password - Apple Support
