Email addresses are used in a multitude of places online, both as a contact method and as an identity. It can be difficult to migrate from one address to another. Here are some suggestions:
Many organizations use Mailman software to send mail to multiple recipients. Each message includes information on how to unsubscribe. You would also have had a password and instructions mailed to you when you originally subscribed.
If you know your password, you can use your existing subscriber address and password to login to a user portal such as http://lists.some.org/mailman/options/some-list. You can then change your address while keeping your subscription.
If you do not know your password, you can request that it be reset. For this, your existing address must be active - you will be sent a new password.
If your subscription address goes inactive for too long (too many messages to you are rejected), you will be automatically unsubscribed. Alternatively, you may email the list administrator, or just re-subscribe with a new address.
Other mailing list software may operate in a similar manner.
Most social media platforms have a sophisticated system for automated password resets and verification. It is usually possible to change your email address after it has gone inactive, provided that you have an alternative conntact method in place. If you do not, you may have to spend hours on the phone, or mail identity documents such as a driver's license. Set up alternates beforehand. Some sites have both a login address and contact addresses, possibly in different places.
Twitter has a "Settings and Privacy" link under your avatar top right. There is only one email address. If you forget your password, you need either an active email or SMS number, or a device that is still able to login. See Updating your email address
LinkedIn also has a "Settings and Privacy" link under your avatar top right. You may have multiple addresses, of which one is the primary. Providing you still have access to your account, you can easily change these. You may also add a cellphone number to allow password resets via SMS.
Facebook has a General Account Settings link under a tiny triangle top right. You may have multiple addresses, of which one is the primary. Providing you still have access to your account, you can easily change these. You may also add one or more cellphone numbers to allow password resets via SMS.
Google has a My Account link under your avatar top right. There is an email page listing the main account, contact and recovery emails. You can't change the main account email, but you can change the others.
The following websites may have your email address as a contact or password reset:
Many websites use your email address as a login identity. If you use the password manager in Firefox to remember logins, you can get a list of these under Preferences/Security/Saved Logins (pseudo-URL about:preferences#security - you can't click that, but you can copy/paste it). If you have a very large number, and want to get a listing in a text file, you can use the Firefox Decrypt tool, which will extract the data from your browser profile.
Note that while some sites use your email address as a username, many do not. You will have to manually login and go check. The list of saved credentials is a good place to start. For some web forums, you may not be able to change your address; you may have to unsubscribe and re-subscribe with a new one.