Yeah, that should be easy with the NBT editor, haven't tried it though.
You can always try the manual way which is explained on the Minecraft Wiki:
Regional difficulty is a variation in difficulty calculated based on several factors, including the “inhabited time" of the current chunk, the total play time in the world, the phase of the moon, and the current difficulty setting. The current regional difficulty is shown on the Debug screen as “Local Difficulty".
The “inhabited time" of a chunk increases for each tick a player spends with the chunk loaded. This is a cumulative measure of time—if 50 players spend a single hour in a chunk, it counts the same as if one player spent 50 hours there. The effect of inhabited time on regional difficulty is capped at 50 hours. Total play time in the world also contributes to regional difficulty. This effect doesn't begin until after the first hour of play time, and is capped at 21 hours.
If the chunk is not buggy it should max out again. Test and make sure that the local difficulty can change.
Rollback Post to RevisionRollBack
I would love to change the world, but they won't give me the source code!
Simply setting the world time has little effect on local difficulty because the main factor is the inhabited time - and that is only affected by how long a chunk has been loaded, independent of the world time. This is also mentioned in this bug report claiming that local difficulty is affected by /time:
[Mojang] Dinnerbone (Nathan Adams) added a comment - 30/Jul/14 10:07 AM
Changing the time of the world will move it between different phases of the moon cycle, which will affect global difficulty. As far as I can tell, no other influence happens to the actual local difficulty.
The total time spent in a world does have an effect but is only 25% of that of the inhabited time (that is, the total difficulty is 0.75 plus 0-0.25 for the total world time plus 0-1 for the inhabited time plus 0-0.25 for the moon phase; thus inhabited time is twice as significant as the other two variables combined, and after it reaches 4.0 (the maximum on Normal, after adjustments) there is no further effect, even though can reach 6.75 on Hard; see here for details).
As for using a utility like NBTExplorer, that is impractical since you'd have to manually change the inhabited time for every chunk - and that's a lot of chunks; a fully zoomed map (2048x2048) is 16,384 chunks and I don't know if you can change the value for every chunk at once, either with NBTExplorer or another utility.