fitting mode S
The aerial is the same so you can re-use that as to the tray , it depends on the unit.
Some King units can do a direct swap BUT and this is the BIG BUT !, the new tray has a mount for an E-PROM this has all the unique aircraft data and lets you swap units from aircraft to aircraft. If you don't have the E-PROM when you swap units it will require you to enter the unique code into the box before flying.
In my opinion the extra hassel in changing the tray is worthwile in the long run.
Narco are at the moment looking into making a mode S unit to replace the AT50/150/155 range and acording to the people at Narco that I have emailed the plan is to have the unit fit into the same tray , this would please me no end as I own three of these units !.