Whilst pre-para (for RAF Regt) and P-Coy (for Army) are requirements put in place by those organisations for attending the parachute course at BZN it has never been a requirement of the course itself. A certain level of fitness is expected and units that sent folk more suited to MSP delivery would be looked upon in a negative light but there are a few units out there (such a TSW, TCW and a few other mainly RAF units) that have a scaling for jump courses and funded slots due to a theoretical war role.
The wearing of the 'lightbulb' is linked to the funding behind your acceptance onto the jump course. If you are attending to qualify to fill a funded establishment slot (parachute pay payable) then the full wings are the correct qualification badge and this can be retained on leaving the role for which it was awarded. If though an individual attends the jump course as an unfunded attendee with no established parachute role in current post then the 'lightbulb' is the correct badge to be worn. If subsequently though the individual moves to an established parachute role then the 'lightbulb' should be replaced by the full wings.
To sum up, the type of badge initially awarded is based purely on whether the individual is in a funded parachute role. Subsequently the 'lightbulb' can be upgraded if individual moves to a qualifying role but the full wings are always retained once awarded.