Aircraft on MyFlightbook are shared among pilots on the system, as discussed in my previous post on this topic.
But what happens when a tail number (i.e., registration) is re-assigned from one aircraft to another?
Here are the rules I follow:
If you are adding an aircraft to your account that is already in the system, MyFlightbook ignores the model you specify on the assumption that the existing definition which has been vetted by other pilots, is correct. Most of the time this is the right behavior: you'll specify a vanilla C-172 but it's in the system as the more specific C-172 N, for example. MyFlightbook sends an email about this to let you know which model was assigned if you are using an existing aircraft.
Once an aircraft is in your account, you can edit its model (web only) by clicking the pencil icon.
If you make a "minor" change to an aircraft, then the underlying aircraft is changed and all pilots with that aircraft in their system receive a notice of the change, since the changed aircraft is used in their account too. What constitutes "minor"? I use two criteria: (a) you're not changing the category/class (e.g., changing from a C-172N on wheels to a C-172N on floats), and (b) you're not changing the ICAO model being used. So, for example, all C-172's have an ICAO of "C172", so changing a C-172 P to a C-172 S is a minor change.
If the change, however, is not minor, then the system will automatically clone the aircraft using the new model and put the clone in your account. So, for example, if you flew a C-172 S years ago but now the tailnumber is assigned to a Boeing 737, you could edit the 737 to be a C-172 S and a copy of the aircraft will be created that shares the same tail number but is a Cessna instead of a Boeing. A notification will again go out to other pilots alerting them to the new version, but nobody else's aircraft will be affected.
Sometimes, when an aircraft is cloned, you actually want both versions available to you. A good example of this is an aircraft that is on floats part of the year and wheels for the remainder. This is not a problem. When you view details for the aircraft (using the web), you will see both versions of the aircraft and you can either switch from one to the other, or you can add an alternative version to live side-by-side with the original.