even if you have vfr details in eg a flight plan it doesn't mean anything to centre. Apart from a class c or d dept point they only hold ifr flight plans. Try setting a sartime on centre they will do it but duplicate your details as they have nothing on your aircraft. As far as i was led to believe from atc they prefer notification of an upgrade via flightwatch as was previously said sooner rather than later and then contact atc 5 mins later after flightwatch have passed details to them