The Belgian AIP has info about delaying but is silent as far as I can tell bringing forward flight plans. It does have info specific to VFR flight plans. ENR 1.10 seems to be the place to look. In this instance I imagine the VFR flight plan was filed because of a requirement (as stated in AIP) to do so because ATC services are required ( ie departing from a controlled airfield) and this is why it is less flexible than a VFR flight plan filed solely for SAR purposes.
https://www.belgocontrol.be/html/bel...dex-en-GB.html