You have to be smart about it.. obviously if you want to make a map change to an area of a live shard, you have to either A) arange for the area to be cleared on the shard first, B) make the changes in a way that exsisting areas wont be efected, like same Z levels in the areas containing houses, etc.
If you just want to add a cave or somthing, make the new cave in an area that you can create a box around easily, to save yourself some trouble. Meaning, you can copy that one area from your new map/statics files created, to the existing live map if needed, and nothing else will be changed, but the programs only support Rectangle areas.
Any static items, buildings and all of that can be untouched. Use World Copy Map, or whatever (even though I use them a lot, I never can remember the actual names of the two similar programs)
By planning ahead making a cave in a box type area, it will save yourself a lot of hassle when using the Map Copy program. You will be able to make just one copy/paste and touch up with WF
