That's right. You need a name on the application to ensure it's approved.
If there is a query about the staff going, add the potential ones but don't cross any safety lines.
eg. Smith, Jones, Bloggs and Doe are all listed, but only Smith and Jones can make it in the end. Make sure you numbers don't pass the threshold for supervision, and that the remaining staff still have (if required) the necessary qualifications.
If it ends up being covered by another Sqn, get email clearance from your Wing HQ in lieu of a change to the pre-authorised application. If necessary document their qualifications.
If it's a local activity you do what you need and then update the system afterwards.
Don't forget, the online systems are requests for permission to undertake an activity. Change the activity in some way and you render your request invalid - otherwise what's the point of it? The only changes you can make are to cadets, recognising the transient nature of cadet attendance.
If you are having to change staff all the time, you might have to start questioning do you have the right buy-in from the team? Odd changes are normal in the grand scheme of things.