OK, what you could do is this:
1. Login to LrM using the NEW account
2. Create a new (empty) album.
3. Tap on the tri-dot icon for that new album and select "Share & Invite"
4. On the "Share & Invite" screen, tap on "Get Shareable Link"
5. Leave Link Access at Invite only
6. Tap on the Invite People item, enter the email address of the OLD account and tap on the icon to the right of the email box and tap on "Can Contribute", then tap on the Send button top right.
7. Find the email and accept the invitation.
8. Logout of LrM, then login in using the OLD account details.
9. The shared album should appear in the "Shared with You" section of the Shared album list.
10. Now you can tap on All Photos, tap on the tri-dot icon top right, tap on Select Photos, then in the Select Photos dialog tap on the checkbox in front of All Photos (which will select them all).
11. Then tap on the "Add to" button bottom left, then select the shared album from above. That will add every photo to the new shared album, and in the following dialog box make sure you check all the boxes to include all the metadata.
In the background, all those photos from the OLD account will be copied to the shared alnum in the NEW account. As the "owner" of the album in the NEW account, you will have full access and ownership of the contents. However, you will have lost any album organisation that you may have had in the OLD account, though of course you could do this process on a per album basis if that is practical, rather than doing All Photos in one hit.