There is no flight plan on earth which can not be cancelled and refiled. Actually you can do it with a simple SITA message to CFMU if you know the correct format and address to use. Within 30 secs you can cancel/delay any (live) flight plans of the world (belonging to any company) if you know the proper flight number and dep/dest station as CFMU is not checking the authority of the sender only the message format. You will be later catched by your address but that's a different story.
However if you refile a flight plan you will automatically be put at the very end of the queue of flights using the limiting airspace/aerodrome (being an LF = Late Filer) therefore it is the worst tactics to fight against a slot. For a simple 10 mins extension your OPS guys can call CFMU Helpdesk, they help in 90% of the cases without sending any delay message. If you got the sh#&đy CTOT already, better to keep the original flight plan/EOBT, ask ATC to relase a ready message and wait for someone else delaying their EOBT or your ops guys to beg successfully to CFMU helpdesk for an improvement. Let's hope that AENA/UFOENA/WHOKNOWSENA does not prohibit ATC staff to send ready messages...
