My conclusion, as of today, is that it is completely safe to replace the AppID prefix by a new one, with the exception of apps using the keychain. This opens up for migrating the prefix for all my exiting apps on the App Store to the new TeamID based prefix.
This conclusion is based on the following input:
- A discussion I had with Apple's developer support a couple of weeks ago. They told me that there should not be any problems changing the prefix, except for apps using the keychain.
- A test I did, using ad-hoc distribution, verifying that an existing app on a test device did not lose any data when it was updated to a new version with another prefix.
- The fact that I have successfully uploaded new versions, with replaced prefix, of two of my existing apps without any problems so far.
The status of the two "live" apps that I have changed, is that they have been approved in the review. However, they are currently waiting in "Pending Developer Release", since I am waiting for a third app to get ready in order to sync the release with that one.
If I see any kind of problems when they go live on the App Store, I will of course update this answer.
Update:
The two apps were released successfully more than three weeks ago. The roll-out worked as expected and I have not received any user complaints.
In summary, the answer is YES, this is completely safe!