Onceapilot, the original plan can easily be updated for any late notice constraint from NOTAMs etc. But I would hope that, in this day and age, promulgation of such information (e.g. TAFs) is better than it was 15 years ago! Trying to get en-route TAFs for the AARC once almost meant ringing the flying club on my mobile phone and asking them to check met on-line (from the MetO), as the RAF system of fax and signal was working as well as ever....
If an abort aerodrome goes out of limits en-route, or an ATC re-route is directed, the mission plan can easily be recomputed by the operator. Rather more quickly than a trail I recall taking a fighter from Bermuda to the US, spending ages trying to find another abort aerodrome with acceptable weather, then waiting for the AARC (who was with us, fortunately) to replan the trail.
But technology is only a tool which should assist rather than dominate!
Neither does it have the wealth of route experience which mates like TenghaType used to have - such as how to get the best out of '10% Carlos' at Lajes….