If you want to change your Lightroom account to use a new email address, and you also want to keep the existing images, you can simply update the email address your Adobe account uses:
1. go to: https://account.adobe.com/ and sign in
2. click on Change Email, and update your Adobe account to use the email address you prefer.
3. Sign out and sign back into Lightroom to ensure you can still access your images.
If you have already created a new Lightroom account using a new email address, and you want to subscribe this new account instead of keeping the old account/subscription, you must first cancel your original subscription. Lightroom will not let your Apple ID subscribe more than one Lightroom account at a time. If you wish to cancel Lightroom Premium, please see this for more help: https://support.apple.com/en-us/HT202039