Keep a Unicode master
Complete editing and proofreading in Unicode first. Store that file as the durable master, then create a legacy export only for the system that requires it.
A reverse conversion is a compatibility step, not a replacement for standards-based storage.
Verify the destination font
ASCII output only displays correctly with the matching Nudi font and mapping. Ask the recipient which version they use and test a short passage containing vowels, conjuncts, numerals, and punctuation.
Plan the return path
If corrections happen in the legacy application, capture them carefully in the Unicode source. Otherwise two diverging versions can develop, making later archival conversion harder.