My email address I use for the Play Store will be invalid soon, so I need to change it. However I don't want to make a new account because I want to keep all my apps. How can I do that?
You should be able to add an alternate email to your Google account. Afterwards, you can sign in with any of your alternate email addresses, using the same password for your main address.
Once your current email is invalidated, you should also be able to remove that address from your Google account and continue using your alternate email as the primary login.
Since the Google account stays the same, all of your apps will remain on your account.