Having a quick flick through EU-OPS the only real bit I could see that indicates the definition of in-flight re-planning is as follows.
OPS 1.255
Fuel policy
(d) An operator shall ensure that in-flight re-planning procedures for calculating usable fuel required when a flight has to proceed along a route or to a destination aerodrome other than originally planned includes:
My take on this is that in-flight re-planning only occurs if you choose to fly a different route to get to your original destination or you fly to a new destination not originally planned for the flight at t/o. There does not seem to be any comments or references to alternates. Therefore to me it seems that deciding to divert to a different destination alternate other than the ones that where chosen at the planning stage would not be considered in-flight re-planning.
OPS 1.340
Meteorological conditions
2. continue beyond the point from which a revised flight plan applies in the event of in-flight re-planning, when information is available indicating that the expected weather conditions, at the time of arrival, at the destination and/or required alternate aerodrome(s) prescribed in OPS 1.295 are at or above the planning minima, prescribed in OPS 1.297.
(b) On an IFR flight, a commander shall only continue towards the planned destination aerodrome when the latest information available indicates that, at the expected time of arrival, the weather conditions at the destination, or at least one destination alternate aerodrome, are at or above the planning applicable aerodrome operating minima.
As far as in-flight re-planning goes and how it is linked to our subject, I think you have to break it down into the different phases of flight.
En-route:
When en-route for you to be able to continue towards your destination airport, your alternates must meet the planning minima (see quote above). Again if you re-plan to go to a new destination or take a new routing your associated alternates (whether new or original) must meet the planning minima for you to be able to continue with your flight. Where we have been talking about diverting to an airport that has not met the wx minima at the planning stage e.g. CAT II but RVR 400m this is no good when en route and can not be selected as an alternate for you to be able to continue towards your destination.
Over head your destination:
Once over head your destination you now only need plate minima at your alternates. Therefore now if you decide to divert to an alternate that did not meet the wx minima at the planning stage and therefore was not one of the designated alternates for the flight (in my view this does not count as re-planning) you are OK to divert to it with CAT II and 400m RVR (as apposed to planning minima of CAT II require CAT I RVR/550m to be selected at the planning stage)
Would be interesting to know how many people know about this topic. You learn something new each day 
Last edited by big d1; 5th Dec 2009 at 01:23.