Dynamis as we know it will be gone; if it's like Abyssea, that could be awesome as long as the zones can support multiple farming groups.
On the flip side, the storyline type "epic battle versus the trapped Beastman Confederate hordes!" feeling will probably be gone; I do not see a way to "lay siege" to the nightmare zones when the whole zone isn't reserved for your group.
So, instead of having one epic battle of your side versus theirs, you will take part in a much larger battle that constantly rages between the trapped Beastman forces and many groups of adventurers; a perpetual battle which never ends, similar to Abyssea. That can work too, and will likely be just as fun.
With Key Item pop progression, you can even force the same order of kills for certain events (most notably killing the Demonic Nobility to pop Dynamis Lord, or the Vanguard Eye that pops Attestation NMs). While the pulling will obviously be different, and paths through the zones taken by groups to complete the "original goals" (which in this case would require a succession of Key Item pops) will be revised, it is possible (though difficult) to maintain the same challenge as before. This, along with an increase in level for the mobs, will provide a refreshing change to an old event which has needed a makeover for years.
If all of that comes to pass in that way, it will be a great update.