Apps (and any other download from the AppStore) are forever
tied to the AppleID used to download them. AppleIDs cannot be
combined or merged. Either continue to use the old AppleID to
update the apps downloaded with it or delete those apps and
download them with the new AppleID. If they are paid for apps,
you will have to buy them with the new AppleID if you do not
wish to continue using the old AppleID for updates.
Same thing happened to me. Apple MADE me change my Apple ID and password without reason. It was after I got a new phone from warranty and they seemingly couldn't verify my identity, so I was forced to change it up. Now I cant update my apps without deleting and redownloading. Unfortunately, that seems to be the only way. The only thing about it, is previous data from those apps will delete as well.
***** cause I'm on level 179 of Candy Crush. hahahaa