As I understand what I have been told, non commissioned personnel do retain a link with their original regiment and start off as 'Troopers' and are promoted within the Regt after completing selection. The RSM of the Regt, rather like the Academy RSM at Sandhurst and the Garrison Sgt Maj London District, have special terms of service to 'compensate' for not taking a commission and they serve for a much longer period. Officer candidates who pass selection become Troop Cdrs and are ranked Captain. At the end of their tour they go elsewhere but can be invited back as a Major in due course but I suspect that this depends on how they did first time round!
In the Falklands conflict a sqn cdr who was seen/thought not to be keen on attacking the Argentine air base from which Super Etendards operated was removed from command and in Gulf War I, I understand that the RSM was sent to take over from another sqn cdr who wasn't 'pushy' enough. This is all in the public domain, so I'm not 'telling tales out of school'!
Old Duffer