I have two games in Google Play Store and both of those are developed with Unity3d game engine. But I think it is possible to redevelop these two games with libgdx. Keeping the package name same and with new version, is it possible to upload the same game using libgdx?
if you have published an app using a given package name, you can reuse that as long as you have the same certificate. You can do whatever you want to an app in terms of updates as long as you use the same certificate and package names.
For an instance, you could re-create an app from the ground up where the original app was in C++ and the new one is in Java. Using the same signing certificate and package you can then post it as an update. Essentially, the engine is replaced but the app is the same.
So as long as you keep the same package name and certificate you can update the app with a new core (here: changing from unity to libgdx)