Vgr, I'm 100% sure that you're wrong. In Ultimecia's time (the future) you have portals. What those portals are for? Do they serve you to go to the world map? Nope, you could just go to the world map exiting the orphanage, why would you need a crazy thing like portals? Those portals (which in games and movies are almost always related to time-travel) serve you to go back to your time (present), so that you could visit towns, talk to people, get missing G.F.s, fighting monsters, do side-quests, etc. Ultimately they decided to not include towns, maybe because of the space, maybe because of NPC talking, or for other reasons. But with those portals you definitely go back to present time.
I would like to remind you that the time compression was not completed: the plan was that Ellone would bring back Ultimecia (in Adel) before she ended casting the time compression; using this not-completed time compression Squall & co. was able to go to the future using their will.
This is a very common misconception regarding the plot: Squall & co. in the Ultimecia's castle are NOT into a weird time were past-present-future are merged; they simply are in the future. You can see the "real" time compression when the game shows you those crazy FMVs where the charaxcters seem to float in an hallucinogenic trip. Also when you fight the sourceresses. In Ultimecia's castle you're instead in a "stable" future time.
If they can go to the future there's no reason why they couldn't go to the past. To make going to the past clear and scenic they've put portals.
(@DLPB)
For these reasons there wouldn't even be the need to update NPC conversations, IMHO.
The only locations you can still go are the places that you get Guardian Forces from, and the Ragnarok.
Yes, because the player needs those places. Towns have been sacrificed.
Disc 4 is 678 MB BTW, if anyone wants to venture a guess about the "towns space" theory.