I am amazed at the lack of knowledge about radar services and ATC clearances being displayed on this thread, especially by people that should know better.
Firstly Basic Service, Traffic Service and Deconfliction Service are not available within controlled airspace.
In a radar environment, within a control zone it is radar control service. Pilots should always be informed of a change of service, in order that everybody is clear about their responsibilities for separation, vectoring and flying in accordance with the privileges of their licence.
In a Class D CTR, VFR clearances can be given until the visibility at the associated aerodrome falls below 5 km, at which point the ATCO informs the pilot of the visibility and requests from the pilot what kind of clearance he requires. The pilot is responsible for determining his in-flight visibility and can elect to continue VFR, or request a SVFR clearance. At night in a Class D CTR, pilots must fly either SVFR or IFR and will be provided with standard separation from other such flights.
I sincerely hope our military colleagues are fully briefed on such points before summer 2012.