Your question is self explanatory. You have already know the solution.
In the reply of your first point
- we can use same application ID
In your flutter application, navigate to android project's app level build and update the flutter app Application ID with your existing application id. (My be you already know it).
Mobile OS installs the app based on Application ID, So you can not install multiple apps with the unique ID.
Now lets come to the second point
- we can use same account as well as KEYS.
When you will generate the flutter app (android/ios) with the existing keys. Lets say your existing app on google play console is built with xyz.jks. Built your flutter android project with same key and publish on google play store.
But, How to notify existing app users for latest update?
If you built your flutter android apk with the same key (xyz.jks), you don't have to worry about the update notification, google play store will handle it and notify the users that an update is avaiable.
But if your end users disabled the app update notification, he will not be notified. In addition to it, if you change your key (xyz.jks) to build the release apk, all your existing users will not be notified.
Tip: if you have integrated firebase in your apps, then you can send update notification from FCM to the users that an update is available.
Note: I never published iOS app so I don't have info about app store so far.