Changing ones Apple ID can cause lots of issues and you are now seeing why. You are going to have to remove the watch from the Apple ID you used when you set up the watch. If you don't remember it, you can try the help in the links I provide below or if you have the original purchase receipt, which clearly shows the watch serial number on it, proving you are the original owner, you can take that and the watch to your closes Apple Store Genius Bar for help. Without proof of original ownership, Apple won't remove the Activation Lock on the watch for you or anyone else.
Click these links:
https://iforgot.apple.com/appleid
If you forgot your Apple ID password - Apple Support