Old Apple ID
Why is my Ipad asking me to update my Apple ID password for an old Apple ID I do not use?
It may be trying to update something that was downloaded originally with the old account. If you change the account you use, you lose access to all the things downloaded with the old account. Copies will remain on the device and will work, but if you need to download them again or update them then that has to be done using your current account. That may entail purchasing them again unless you set up Family Sharing with your old account and the items can be shared.
Set up Family Sharing - Apple Support