If your primary email address has changed, you simply need to update your AppleID account with the new email address.
Details as to how to change elements of your AppleID account can be found here:
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
You will still need to recover access to your AppleID account - for which you will need your Apple credentials. If forgotten, they can be recovered here:
https://iforgot.apple.com
If you intend to reuse your iPad, you have no alternative other than to recover your original AppleID account - as your iPad will be Activation Locked to this account. Whilst you can create a new AppleID account, the established Activation Lock cannot be resolved without the original account.
Consider also that any paid Apps, Apple Services or Subscriptions are forever tied to the AppleID from which they were purchased - and cannot be transferred.
If the iPad is already Activation Locked, here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Turn off Activation Lock – Apple Support
As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.
If you have the necessary documentation, Apple have recently introduced a new portal though which you may be able to request release of an Activation Lock:
How to remove Activation Lock - Apple Support & start an Activation Lock support request.
Should you require assistance from Apple to resolve issues with your AppleID, this page details points of contact:
Contact Apple for help with Apple ID account security – Apple Support