Inspect the file before altering it
Work on a copy of a subtitle file you may lawfully modify. A plain-text editor can show whether the contents resemble numbered SRT cues or a WebVTT document beginning with its required header. Keep the matching video edition identified as well. Converting a format will not repair a subtitle track written for a different edit, language or programme runtime.
Keep timestamp syntax exact
Amazon’s SRT delivery guidance describes timed blocks using a comma before milliseconds. WebVTT uses its own timestamp syntax and document structure. A punctuation change alone is not a complete treatment of every potential difference, because cue identifiers, settings and styling can also matter. Use a converter that explicitly supports both formats and review a short result rather than applying an unrestricted search-and-replace to the only copy.
Preserve language and readable characters
Check accented characters, punctuation and non-Latin text after conversion. WebVTT is UTF-8; an input file interpreted with the wrong encoding can already contain incorrect characters before the timestamps are touched. Merely saving corrupted text again does not reconstruct the intended words. Keep the original file available so the input encoding and conversion step can be examined independently if the result looks wrong.
Verify timing and presentation separately
Test a cue near the start and another later in the programme, plus any relevant line positioning or formatting. If the whole track has one fixed delay, that is a different task from a progressively growing mismatch. Record the source format, target format and conversion tool. A successful short playback check establishes that particular output, rather than guaranteeing every television application renders all timed-text features identically.
Questions, answered
Can I turn SRT into WebVTT just by renaming it?
No. The document syntax must be converted and validated. Check timestamps, encoding and any cue formatting against the target format, then test it with the matching video.