I'm guessing most comms have things sorted out by now. But I've been doing a bit of experimenting, and I thought I'd put my results here in case they can help or reassure anyone else.
So, first of all - any posts already in a DW comm when you do an import from LJ will not be wiped. That is, posts directly posted to the DW comm or copies of LJ posts from an earlier import - once they're there in the DW comm, they are there permanently unless you choose to delete them. No future imports will affect them.
This also means that if you import a post from LJ to DW, and then that post is deleted on LJ - its copy will still remain on DW for all following imports. (And if you delete an LJ copy on DW, you do have the option of reimporting it, as long as it still exists on LJ.)
All new comments can be imported. If an LJ post is imported to DW and then another comment is added to the LJ original, that comment can be imported over to the DW copy. And no DW comments will be affected by the imports.
In fact, the software only ever imports new stuff over to DW, which is a good thing. You won't end up getting multiple copies of posts. The only drawback is that if an LJ post is edited on LJ after it has been copied to DW, that edit won't be picked up in the next import. The original and its copy have to be edited separately. (Or the copy on DW has to be deleted altogether and the LJ original imported again.)
You cannot of course crosspost from DW comms to LJ comms - you have to copy and paste if you want a modpost, for example, on both sides. I haven't directly tried this, but I would assume the software would count these as two separate posts - even though visually they would be identical. So I guess those kind of 'twin' posts would be counted as new posts and would be imported across. But you would have the option of deleting those if you wished.
Here are the experiments I did:
1.
I put a test post on the new DW Holmes Minor comm.
I put a test post on the LJ Holmes Minor.
I did my first import, and checked the DW post remained. Which it did ^^
2.
I deleted the copy of the LJ test post that was now on DW.
I changed the text slightly in one of my posts on LJ.
I added a test comment to another of my posts on LJ.
I reimported the LJ Holmes Minor.
I checked that the LJ test post was reimported - Yes!
I looked to see if the edited post was - No.
I looked to see if the extra comment was - Yes!
3.
I left the LJ test post in situ on DW.
I deleted the LJ test post on LJ.
I added a comment and reply to an imported post on DW. And added a different comment and reply to its original on LJ.
Once again, I reimported the LJ Holmes Minor, and looked to see if the DW version of the LJ test post stayed - Yes!
I checked that the LJ comments were imported over, and that the DW comments were unaffected - Yes!
So, first of all - any posts already in a DW comm when you do an import from LJ will not be wiped. That is, posts directly posted to the DW comm or copies of LJ posts from an earlier import - once they're there in the DW comm, they are there permanently unless you choose to delete them. No future imports will affect them.
This also means that if you import a post from LJ to DW, and then that post is deleted on LJ - its copy will still remain on DW for all following imports. (And if you delete an LJ copy on DW, you do have the option of reimporting it, as long as it still exists on LJ.)
All new comments can be imported. If an LJ post is imported to DW and then another comment is added to the LJ original, that comment can be imported over to the DW copy. And no DW comments will be affected by the imports.
In fact, the software only ever imports new stuff over to DW, which is a good thing. You won't end up getting multiple copies of posts. The only drawback is that if an LJ post is edited on LJ after it has been copied to DW, that edit won't be picked up in the next import. The original and its copy have to be edited separately. (Or the copy on DW has to be deleted altogether and the LJ original imported again.)
You cannot of course crosspost from DW comms to LJ comms - you have to copy and paste if you want a modpost, for example, on both sides. I haven't directly tried this, but I would assume the software would count these as two separate posts - even though visually they would be identical. So I guess those kind of 'twin' posts would be counted as new posts and would be imported across. But you would have the option of deleting those if you wished.
Here are the experiments I did:
1.
I put a test post on the new DW Holmes Minor comm.
I put a test post on the LJ Holmes Minor.
I did my first import, and checked the DW post remained. Which it did ^^
2.
I deleted the copy of the LJ test post that was now on DW.
I changed the text slightly in one of my posts on LJ.
I added a test comment to another of my posts on LJ.
I reimported the LJ Holmes Minor.
I checked that the LJ test post was reimported - Yes!
I looked to see if the edited post was - No.
I looked to see if the extra comment was - Yes!
3.
I left the LJ test post in situ on DW.
I deleted the LJ test post on LJ.
I added a comment and reply to an imported post on DW. And added a different comment and reply to its original on LJ.
Once again, I reimported the LJ Holmes Minor, and looked to see if the DW version of the LJ test post stayed - Yes!
I checked that the LJ comments were imported over, and that the DW comments were unaffected - Yes!