If a user installs an app with some permissions in Lollipop, and then updates to Marshmallow,
will the permissions of the app be granted or will them all be revoked by default and permission granting window be needed again?
The app is build with Android M as target sdk
F.E. My app has a process that runs on the background and requires a permission, the Lollipop user installs the app and it goes ok, but then updates the phone to Marshmallow.
Will the app process fail until the user grants the permission?
The app is build with Android M as target sdk
Yes, you need to update your app to request the user for the permissions as required by your app otherwise your app may crash.
If your app's target sdk would have been less than 23, then your app would work without any issues.