First: one simplified form, several traditional ones
Simplification collapsed several traditional characters into one simplified form. Going back the other way, one simplified character corresponds to several traditional ones, and choosing between them depends on what it is paired with:
Swapping character by character collapses them all into the same one and gets every instance wrong. Resolving by word is what separates them — the tables come from OpenCC, matching phrases before single characters. For example:
- The "clean" sense maps to one traditional character, the "tree trunk" sense to another
- The "hair" sense and the "development" sense of one simplified character go to two different traditional ones
- The "behind" sense takes one traditional form, while "empress" keeps the character it already has
Second: Taiwan and Hong Kong are not the same thing
There is one more layer after converting to traditional: the same word is written differently in Taiwan and Hong Kong. The word for "software" is one thing in Taiwan and another in Hong Kong, and Taiwan uses its own word for "information".
Hence the separate directions. Taiwanese traditional even splits into "change wording" and "glyphs only" — glyphs only keeps your existing vocabulary, while change wording swaps regional terms too. Hong Kong gets a direction of its own.
Third: only dialogue should be touched
A subtitle file is not only dialogue. Index numbers, timings and ASS style lines are in there too, and not one character of those should be converted — ASS style lines especially, since they contain English directive names that break the moment they are treated as dialogue.
So conversion happens after parsing by format: SRT, VTT, ASS and TXT each identify which part is dialogue, and only the characters inside it are converted. Timings and styles survive untouched, with no formatting to redo.
Going back to simplified: watch out
Converting traditional to simplified also needs you to say which traditional variety you are coming from: generic, Taiwanese and Hong Kong are three different routes, because regional vocabulary has to be restored to the generic word before it can be converted. Pick the wrong direction and regional vocabulary survives into the simplified output.
The lower group of the nine directions is the one that converts back: traditional → simplified, Taiwanese → simplified, Hong Kong → simplified.
Related tools and guides
The full walkthrough — How to convert simplified and traditional subtitles; if all you need is simplified to traditional — Convert simplified subtitles to traditional; to translate into another language, Subtitle translation.