Paul Farquhar
Legend
One example of a subclass that breaks rule #2 "without gaining abilities that you stop using" is the Battlerager from SCAG. Interpreted RAW, if you wear the spiked armor required to use most of the subclass abilities you cannot use the core class ability "Unarmored Defence".
Now, I believe that most people allow the UAD armor class to be used if it is better than the armor class you would get from the armor you are wearing, but RAW wearing armor deactivates the power.
Now, I believe that most people allow the UAD armor class to be used if it is better than the armor class you would get from the armor you are wearing, but RAW wearing armor deactivates the power.