2 iTunes accounts, and a constant need to reauthorize my access

I somehow ended up with 2 accounts on iTunes many years ago, but authorized my MacBook to access both and have maintained the passwords for the old one even though no new purchases have been made on it for years. After I recently installed the latest updates to Music, I now regularly have to reauthorize my Mac to play the music from the old account. Each time it tells me afterward that the computer has already been authorized, but each time it stops playing music until I go through the process for any songs associated with the older account. How do I resolve this? Is there a way to merge the accounts? Or move all of the songs from the old account to the active one?