Thank you for taking the time to reply @Jeff-UK 
I tried what you suggested, using a different browser (Chrome now, Firefox before). For each login-attempt with either the UserID or Email I used a separate, private browsing session so that no cookies with login credentials were stored and I always had a clean browser state.
Unfortunately, the issue persists. Regardless of whether I log in with my new UserID (linked to addr-A now) or with addr-A directly, the UserID shown in the eu1 Console profile settings is consistently wrong (it shows the old UserID, which should not be associated with addr-A anymore and definitely not with the new UserID in any way). Everywhere else, like main site and “The Things ID” profile page, the information is correct.
Test results:
thethingsnetwork.org (main site):
Login with UserId: new-user-id
- Displayed UserId (top-right): new-user-id
GOTO My Profile > Account Settings (id.thethingsnetwork.org/client, The Things ID):
- Email in My Profile: addr-A
- Displayed UserId (top-right): new-user-id
GOTO Console (eu1.cloud.thethings.network):
- Email in Profile Settings: addr-A
- UserId in Profile Settings: old-user-id (<- incorrect!)
-------------------------------------------------------------------------------
Login with Email: addr-A
- Displayed UserId (top-right): new-user-id
GOTO My Profile > Account Settings (id.thethingsnetwork.org/client, The Things ID):
- Email in My Profile: addr-A
- Displayed UserId (top-right): new-user-id
GOTO Console (eu1.cloud.thethings.network):
- Email in Profile Settings: addr-A
- UserId in Profile Settings: old-user-id (<- incorrect!)
-------------------------------------------------------------------------------
Login with UserId: old-user-id
- Displayed UserId (top-right): old-user-id
GOTO My Profile > Account Settings (id.thethingsnetwork.org/client, The Things ID):
- Email in My Profile: addr-B
- Displayed UserId (top-right): old-user-id
GOTO Console (eu1.cloud.thethings.network):
- Email in Profile Settings: addr-B
- UserId in Profile Settings: old-user-id
-------------------------------------------------------------------------------
Login with Email: addr-B
- Displayed UserId (top-right): old-user-id
GOTO My Profile > Account Settings (id.thethingsnetwork.org/client, The Things ID):
- Email in My Profile: addr-B
- Displayed UserId (top-right): old-user-id
GOTO Console (eu1.cloud.thethings.network):
- Email in Profile Settings: addr-B
- UserId in Profile Settings: old-user-id
Summary:
As you can see, everything seems correct on the main site and The Things ID, but the eu1 Console still shows old-user-id for addr-A.
So it seems like the identity server has the right associations (addr-A → new-user-id, addr-B → old-user-id), but the Console is out of sync and stuck showing the wrong (old) UserID.
Is this a known sync issue between the ID server and the Console, or is there a way to force the Console to refresh its UserID mapping?
Oh and one more thing: On every site my old-user-id is displayed as old_user_id (my old UserID has underscores), except in the Console, where it is shown as old-user-id (hypens instead of underscores). Not sure if that’s relevant, but I thought I’d mention it just in case.
Thanks again for your help!