Clearly theres some confusion here, so its worth asking the question "is there a difference between the planning phase and dispatch"
Secondly if a new prob 40 forecast is issued below the normal minima prior to the EEP then you'd need to refer to your specific company docs. Prudence would suggest you seek another alternate but at the EEP if the actual conditions are above the airfield alternate minima, where I work im legal to continue, if they subsequently fall below minima its irrelavent.