I’m really disappointed that all 50,000+ photos have been lost in moving from my xxxxxxx@gmail.com account to xxxxxxxx@msn.com.

If your subscription is with Adobe you should be able to sign in to your Account profile at-

https://account.adobe.com/  and change your email address there on the right of page-

You may need to sign in with the gmail address as your ID. If verification wants to use the gmail address that is no longer available to you, you may need to select another method of verification on the ‘Sign In’ dialog. Then change your account profile to update to the .MSN address.

Your photos should still be there in Lightroom.

Seek Customer Support if this procedure does not work for you-

! CONTACT CUSTOMER SUPPORT   Open a chat window from the chat icon (lower right). Typing “Agent” will get you past the ‘AI answer bot’ to a human.