Currently, there is no way to reset the UCS to WCS. Oh, you can create a UCS that duplicates the WCS, but then you are stuck with a UCS that you can't get rid of until the user manually goes back to WCS.

Also, after opening a drawing, if there has been no user access to the UCSs, a call to ActiveUCS will fail, even though there must obviously be a UCS, even if it is just WCS.