I have a DTdb with some 900 documents that have been extensively replicated to other groups. I would like to have these files indexed (as opposed to imported) as I tend to periodically edit these documents and want to avoid having multiple copies of the same document.
You do not need to change to Indexed files to accomplish what you wish.
You may edit a file that has been copied into the database (Imported) simply by opening it under its parent application (Open With, or in the ToolBar, Open Externally), making changes, then pressing Command-S (Save) to save the changes back to the database. And of course in the case of plain or rich text notes created within the database you may edit tham as well, using DEVONthink’s text editor. This does NOT result in another copy of the document; you have modified the original document. (Do NOT use “Save As”, which will create a second copy of the document.)
If the document that you edited and saved was a replicant, all other instances of the document will be updated. That’s what replicants are all about. 
Thanks Bill. Does that not presuppose that I work primarily from within DT? If I happened to open the document outside of DT with the original application (e.g., Keynote), are there not two “versions” that exist, the original file on my HD and the unedited version within DT?
Yes, it does presuppose that.
Once I’ve captured a file from the Finder I have no further interest in the original and may delete it or move it to an archive disk. With my backup strategy the contents of my databases are well protected.
The ability to use replicants is one of several reasons for my preference for self-contained databases. Other users may have valid reasons to prefer Indexed databases.
I don’t know any approach that could change your database from Imported to Indexed, without messing up your use of replicants.
If you export the current database contents to the Finder, then Index the exported files into a new database, replicants will become duplicates. But if you start over and Index files from the Finder, you will have lost the results of your previous organization efforts, and you will experience less freedom to reorganize content.