If the old system was Fox+ or Foxpro, you could use IDX files, but Clipper was different. They had an NTX (or was it NDX?) index file that wouldn't work with Foxpro. The DBF's were fine - you could add, remove, etc. It's just those clipper index files that couldn't be updated from Fox. Adding data would clobber the indexes on the Clipper side, but they'd never see it. The entries added through Fox wouldn't appear if an index order was set.

Back in the day we used the Comix RDD (replaceable database driver) which gave us Foxpro compatible CDX's on the Clipper side of the equation. That was the only way we could ensure index integrity between VFP and Clipper. It worked like a champ up until last year when I retired my very last running Clipper app.