I believe that alternate requirements apply in flight as well. That is, if you get an updated TTF which has WX conditions below the SPAM / ALTN minima, you must provide for an alternate or divert to an airport where fuel can be picked up or if that's just not possible, continue with a PAN call.
Can anyone tell me where it says alternate requirements don't apply when airborne ? (Don't have the books with me here)
Some of us can probably still remember when ATC OPS would issue operational weather requirements to airborne aircraft that forced them to comply with the steps above. (Showing my age now!)