You're running a server and you didn't take a backup?
On the subject of IDs - doesn't Minecraft allocate them nowadays, removing the need for us to have to worry about them? Does this mean that that system is a little less robust than we'd like?
MC1.7 does indeed automatically assign IDs now, the names are persistent but the IDs are auto assigned (on first startup). That being said unless there are major changes to existing mods, it should maintain the original IDs in the world folder.
We have tested this on UnionCraft, purging a mod from a pack will not shift IDs, it will just remove them from an existing world.
I think the problem is mainly due to the mixed bag nature of 1.7, we are still in the process of shifting over to the new methods as plugins (mainly) still use IDs and as server admins were still used to using IDs instead of names, there is no doubt the changeover was a big benefit to mod devs which is great!, but for admins its definitely a shift away from ease of use.
Of course, a world backup before any update is a no-brainer and is noones fault except the server admins.
I will admit tho, server setups on 1.7 take a significant amount of extra time and work to accomplish.