If you had an FSA stem, it was never thread stem, I think the question you have asked is a bit confusing, as its talking about standards incorrectly.

What most people understand when it comes to headsets and associated parts (caps, stems etc) the words 'threaded' and 'threadless' equal two diferent types of headset, which have differnent stem types, whcih are not interchangable, i.e. when describing anything to do with a stemas threaded, you would think quill, for threadless, a-headset.

From your 2nd post, I think that the threaded part you are refering to is actually the fork bung / expander.

Some photos / exact parts you are working with (fork, and stem model) would be useful here.