After my KitKat update, I still have my old texting app, but it won't work. If I inadvertently select that one from the options-- the text or MMS refuse to send. Is there a way to remove it? Or, change so that I can send from both?
Hangouts takes over texting if you enable it. My honest suggestion is to keep SMS/MMS in the default texting app as it is a bit better equipped for that, and use Hangouts for IM if you even use it for that.
You may be able to disable the default Messaging app in settings if you want to go the other route. To remove it, you may have to root your phone. You can't just uninstall those apps. They're built into the FW.