Watch setup keeps asking me to sign in with old id

I wanted to update my Apple Watch (series 3) because it keeps saying it can't install an update due to lack of memory, no matter if I delete all applications. The recommended strategy was to unpair and setup as new, then unpair again and restore from backup.

So I went along and unpaired. Upon pairing you need to sign in with your Apple ID.

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It uses an old id of mine. A while ago I changed my apple id from rdouma@unseen.is into my current email (unseen.is doesn't work anymore). I have no way of modifying this here. I can do "Forgot your password", then "Forgot my id", then provide my details, then it confirms that is my Apple ID, then I can go back to logging in again and it again shows the old id.

I checked my settings in the App Store: it's the correct email address.

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I checked my settings in Settings → Messages; also looks correct to me.

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I went to Apple Support, but when I enter my serial number, the only option available to me is to bring in my watch for repairs or something.

What do I do?

[Image Edited by Moderator to Remove Personal Information]