Ok, I'll bite.
The AIP provisions for alternate airfields make no distinction between planning and in-flight cases. Therefore, I believe a straightforward interpretation is that the provisions apply for all stages of a flight.
Ref: Jepps AGA Sect 3 para 3.2.1
Except when operating an aircraft under the VFR by day within 50 NM of the point of departure, the pilot-in-command must provide for a suitable alternate aerodrome when arrival at the destination will be during the currency of, or up to 30 minutes prior to the forecast commencement of, the following weather conditions:
Nothing about planning, just "operating" and "when arrival at the destination will be..."
Cheers,
O8