On the calendar view when dragging and dropping your appointments, a pop up will appear to let you know if this move is supported by your availability rules or not. See video below...
For example, see here when I try to move the time of this appointment, it let's me know that the time I have chosen to move this appointment time has no scheduled availability at this time...
This warning let's me know that the practitioner I'm trying to move it to isn't available...
If everything is good with the move you are trying to make, then it will let you know all is good to be moved...
If the warning pops up letting you know there are issues with this move i.e the practitioner isn't available at that time or the time slot is not usually scheduled for this time, you will be able to override this if you have the reservations permission to be able to override booking rules, such as practitioner or room availability when managing and submitting orders i.e. spa manager. You just have to select "Move appointment".
Please note, if the practitioner you are trying to move the appointment to is not set up as able to take the appointment, even if you have the ability to override, this action will not be possible because the practitioner cannot take this appointment. You will need to edit the appointment settings so that the practitioner is listed as a valid practitioner for the appointment.