Changing between IFR and VFR

Cancelling IFR (going to VFR)

Only valid when the PIC transmits the specific phrase "Cancelling my IFR flight", together with any necessary changes to the current flight plan, to an ATS unit.
ATC does not invite this change, directly or by implication — it is the pilot's decision alone.
ATC's only expected reply is an acknowledgement such as "IFR flight cancelled at (time)".
The ATS unit that receives the cancellation informs all other units the IFR flight plan was addressed to, except any it has already flown past.

Changing from VFR to IFR

If a flight plan was already filed, submit the necessary changes to it before or as you make the change.
Within controlled airspace: fly the IFR semicircular cruising levels (SERA.5020).
Outside controlled airspace, above 3000 ft AMSL or the transition altitude (whichever is higher): also fly a semicircular level appropriate to track, using 1013.25 hPa unless the competent authority requires otherwise (SERA.5025).

Source: ICAO Doc 4444 (PANS-ATM) 4.8 / SERA.5020–5025