You are unable to update or transfer the iOS apps present in your single account. You need to republish the apps otherwise Apple will take them down, but you are unable to update or transfer the apps.
You are unable to transfer apps because they are too outdated, and you are unable to update them because the developer account is your own, instead of the client's developer account.
You can talk to Apple Reviews Team and plead your case.
They may reinstate the apps and allow you to republish them using your existing account so that you can get them all updated, and then move the apps to the clients' accounts.