I made some test locally on my laptop (did not changed ANYTHING on the CC MC server), starting from an exact (local) copy of the current CC MC server, here is the results of my testing:
Test 1: Installing a brand new 1.2.5 server with MCForge, Redpower and IC2 installed as mods, copying the CC server world to it. I then started the local server and navigated the world in-game.
Result: works fine, Mystcraft and Buildcraft blocks and items just vanished from the world, without conflicts as it seems.
Test 2: Installing a brand new 1.2.5 server with MCForge, Redpower and IC2 installed as mods, copying the CC server world to it but with all regions files deleted (to force a regeneration with the same seed). I then started the local server and navigated the world in-game.
Result (same world portion):
Current MapRegenerated MapTest 3: From a full copy of CC MC server, trying to delete chunks from the world: Using MCEdit, I selected chunks and used the "prune" command (prune = erase everything but the selected chunks). I then started the local server and navigated the world in-game.
Result: the selected chunks were correctly preserved (including chests content), all other chunks are regenerated as we navigate around, most landscape is the same (using current seed). There is some difference (check the maps of test 2 for comparison) but if the preserved chunks are carefully picked, it should not be a big problem assuming we keep the same seed.
Test 4: From a full copy of CC MC server, trying to "re-populate" (MCEdit "re-pop" command) some chunks with resources: Using that command, it is possible to regenerate the ore blocks within a chunk.
Result: In the selected chunks, some non-empty blocks are replaced by ore (including mods ores!), gravel or dirt blocks; there may be more changes, but I did not noticed anything else.
Test 5: From a full copy of CC MC server, changing the spawn coordinates using NBTExplorer software: I updated the level.dat file by setting spawn coords to 0,66,0 (x,y,z coords).
Result: After deleting some player data files, I was able to force a respawn (just as if I connected for the first time to the server) and i correctly respawned in the vicinity of (x=0,z=0) so it seems to work.
Note that every map editing was made at the chunk level and by deleting chunks from an existing world. The reason is I felt the MCEdit 2D interface to be less cumbersome to use; the 3D interface allow to make custom selection (i.e. any volume of blocks) but the world must be navigated in 3D, which take a bit of practice to do. As time was the essence, I needed a quick way to do testing, so I stayed at chunk level.
Test conclusions:
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From test 1: It seems that removing mods would not cause the map to become invalid, as the affected blocks and items will simply be removed at the first chunk load, so there seems it is not necessary to restart from scratch because of mods removal.
From test 2: It looks like the current seed would allow to regenerate most of the current terrain, which mean that in most case we would be able to keep or transfer array of chunks without having terrain discontinuities. Exceptions are (so far):
- Carole's mountain would be regenerated as a desert (biome change) but the coastline is the same, which mean that if she want it to be preserved, we can keep the whole biome as it is (outlined area with 0range "o" in the following image).
- Large forest around my "Forest Lodge" would be regenerated as hills, with a different coastline pattern BUT there was already a terrain discontinuity in that zone; and regenerating the affected chunks (outlined area with black "x" in the following image) would solve it. I think I am the only one with something built in that area and I have no problem having that stuff deleted.
Reference image (Marked)From test 3: Deleting chunks from an existing map is possible, as long they are carefully picked to make sure there is no discontinuities between preserved and regenerated chunks. In the image above, I marked all the chunks I picked during test with an "o" of any color; that include: Trerro's Castle, Carole's Mountain, my "bubble",my "Sky Dome" under construction and parts of the railway I built.
From test 4: Resources (including those from mods) can be regenerated in preserved chunks, there could be some unexpected changes as dirt and gravel can be placed as well. I think repopulation should be done only on selected chunks, case by case, according to the "chunk owner" decision.
From test 5: Spawn point can be changed! Which mean it is not necessary to restart the whole world to change its location, we can set it at any other place.
Final words:
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According to the test I made, if we keep the same seed, it is either possible to preserve the whole world and "re-pop" resources only OR to preserve only the chunks where something has been built and re-pop resources in those chunks (other will be regenerated WITH new resources)
It is also possible to create a new world with the same or a different seed but I did not tested the copy/pasting functionalities of MCEdit yet, it seems to be more flexible than working with chunks, but also more complex. Also, we must keep in mind that using a different seed could result in having biome/terrain/cave discontinuities around preserved structures.
In light of that, my choice would be to keep the same seed and edit the existing map to preserve only the chunks where something has been built (basically choice A with the least possible amount of preserved chunks). That way, people who want to keep what they have built will not "deface" the landscape since regenerated surrounding chunks will match preserved ones, and people who want to start from scratch will have modded resources available near the spawn point. We could also imagine to move some structures at a different place in the same world, but I do not have any clues on the feasability of that on a large scale.
-Sam
Added note:
A full map of the world is also available, but curiously, the image does not show up on IE browser...! (I used Waterfox to display it)