For departures from CAS, it is likely that whatever FPL you have filed, IFR or VFR, will have to be managed by the electronic system that produces strips for the controllers. What generally occurs is that these electronic systems that convert AFTN FPL messages to usable ATCO strips have time parameters that result in a strip appearing on the controller's display at a set time prior to departure. These parameters may vary between units. If you want to depart well in advance of your filed EOBT, it is possible that the ATCO will have to search the computer for a plan about which they have no knowledge, find the right one, bring it forward, and then ask the other agencies in the CAS, and in particular the parent unit, if they can bring their records forward as well: if they have the staff to do that. Proactive management of FPLs, e.g. sending CHG or DLA or cancelling and refiling is not difficult any more, thanks to the range of software on offer, and it is far better than hoping that ATC have the time and staff to prioritise a request to depart early for someone who hasn't bothered to manage their FPL.