As far as i understand, you would need to write all 14 subjects to convert your ATPL. If you wish to validate your licence, you will only need to write your airlaw and to the flight test. I think the current legislation says it would need to be on a multi crew turbine. Validation only allows you to operate the types on your licence,no conversions or new types can be added.
This is what it used to be like before part 61, as it keeps changing, (version 33) now i am not sure. Best would be to contact
CAA Website or a local flying school who would be more up to date with current requirements.
Good luck!!!
