Haven't found an automated solution to this problem yet. The easiest work-around I HAVE found is:
1) Copy Assembly (including parts and sub-assemblies) with SW Explorer as you stated above. MUST be to a different directory (See #2c below).
2) Go to Windows Explorer
2a) Select all the the drawings you want to Copy. NOTE: A time saver here is if the drawings are not all in the same directory, but they are all under a parent directory, then Search the parent directory for all *.SLDDRW and then highlight (or copy) them here. If all of the files are scattered in multiple directories, and don't share a common parent directory...then you might as well STOP here since #2c (below) won't be possible.
2b) Drag-and-drop (or paste) them to the new directory.
2c) Temporarily Rename the directory that all of the drawings came from. Hopefully all of the files came from the same directory (so you only have to rename one directory), or they all fall under a same parent directory (which can then be the only directory renamed instead) somewhere in their path. NOTE: The Copied files must NOT share the filename path that you just changed.
2d) Highlight all of your copied drawings. I try to limit it to 50 part drawings at a time if you have more than that, and I usually only open drawings of large Assemblies one at a time.
2e) If you are sure that NOTHING is presently Open in SW (or SW is currently not Open at all), Hit Enter.
3) As each drawing file Opens in SW, you'll get an Error Message saying that the drawing cannot find the referenced part/assembly file, Choose YES to find the file yourself. Hopefully you can easily browse to the copied file (i.e. you didn't significantly change the original filenames you copied).
4) On the next Dialog Box (Dangling Detail Items), just click OK. That usually does fine for me.
5) Repeat process as each drawing file opens.
6) You then have to Save and Close each file (manually).
7) Once all drawing file references have been updated, go back to Windows Explorer and Rename the directory('s) from which the original files were copied from.
Not very pretty, but it gets the job done.
...A side note to make here is that you can usually drag-and-drop (either Copy or Move) entire directories in Windows Explorer that contain "basic" parts, assemblies, and drawings and the references should automatically update (but the filenams stay the same). I haven't gotten up the courage to try this with a large assembly that contains any In-Context relations, because I'm not sure this will work for those (i.e. I think that's what SW Explorer is for)...Or maybe I did try once a long time ago, and found out it doesn't work.)
Experiment with same sample files first (or make sure you have backup copies of the real thing available). Let me know if you have any problems or some of my gibberish above is confusing.
Gook Luck,
Ken Bolen