Android : Update already installed android application gives error "an existing package by the same name with a conflicting signature is already installed"

I made an Android application using intellijidea 14 It was great, I made changes in the project and generated and distributed Apk several times in 2014

But Few day ago when I made some changes and rebuild project (I did not use signed apk ever). And try to install apk on device(Where i have application installed already ), then it gives me error while updating . "an existing package by the same name with a conflicting signature is already installed"

I have tried everything , i never generated apk using any signature (never create or used any keystore) i was just using apk generated in Out Folder.

Now i am unable to update my already installed android app i can not UNINSTALL it as i have a database which has crucial values( whihc i can't afford to loose this data ) as application is already installed on 15 devices

I need to update all 15 devices application how can i make a new apk with old apk signature It's very urgent , my boss is going to kill me , please help.

Please help