Update: Turns out that the version of SQL Server is too old for Desktop 10.6 to connect to (2003, 32 bit - remember when I said all of our stuff is old? ) So it wouldn't matter what Enterprise GDB we use anyway. What we're doing now is sunsetting the old infrastructure and putting in all new stuff, ergo my question.
I may try what you have recommended, Xander - use the older client (10.0 to just dump the XML out and then use 10.6 everything from there on out. As another question - do you know if Export XML Workspace will A) even work with class extensions and B) take the class extensions with the export? Trying to get rid of those nasty things, not keep them around.
Thanks so much for all the help!