This is a guide from a file conversion service telling you when not to convert files. We built ChangeThisFile to make conversion easy, but easy doesn't mean always appropriate. Converting a file when you shouldn't — re-saving a JPEG as another JPEG, converting a PSD to PNG for editing, or converting MP3 to FLAC to "improve quality" — either degrades what you have or gives you false confidence about what you've gained.
Knowing when to leave a file alone is as important as knowing how to convert it. Here are the situations where the right answer is: don't.
Lossy to Lossy: Double Destruction
Every lossy encoding pass applies its own quantization, destroying information that can't be recovered. Converting from one lossy format to another adds a second round of destruction on top of already-degraded content.
JPEG to JPEG — Re-saving a JPEG (even at quality 100) applies fresh DCT and quantization. The output is measurably worse than the input, even if visually similar at high quality settings. After 5-10 re-saves, degradation is visible. The only time JPEG-to-JPEG makes sense is if you're applying edits (cropping, color correction) and the source JPEG is all you have.
MP3 to AAC — Each codec discards different frequency information based on different psychoacoustic models. The second codec wastes bits encoding artifacts from the first codec as if they were real audio content. MP3 to AAC conversion is still useful for device compatibility, but set the output bitrate higher than the input to minimize additional loss.
H.264 to H.265 — Re-encoding video between lossy codecs adds generation loss. If the goal is smaller files, the compression gain from H.265 may outweigh the generation loss — but only at the original resolution. Downsizing quality to gain compression is a net loss.
The rule: if you must go lossy-to-lossy, use a higher bitrate/quality setting on the output than the input, and accept that some quality is lost. If possible, go back to the lossless original and encode from there.
Already in the Optimal Format
Some conversions are pointless because the file is already in the best format for its purpose:
- WebP images on a modern website — WebP is already the current standard for web image delivery. Converting to AVIF gains 10-20% smaller files but requires fallbacks for older browsers. If your site works fine with WebP, the marginal gain may not justify the complexity.
- H.264 MP4 for general video sharing — MP4 with H.264 plays on every device and every platform. Converting to AV1 or H.265 saves space but may not play on older devices. If compatibility matters more than file size, H.264 MP4 is already optimal.
- UTF-8 text files — A UTF-8 plain text file is already in the most compatible text encoding. Converting to other encodings (Latin-1, UTF-16) only reduces compatibility.
- PDF for document distribution — If you have a PDF and the goal is to share a finalized document, it's already in the right format. Converting PDF to DOCX and back to PDF adds two conversion passes and likely introduces formatting errors.
Source Files: Keep Them Native
Source files contain editing information that's lost during conversion. Never convert a source file as a substitute for keeping the original.
| Source Format | What's Lost on Conversion | Keep For |
|---|---|---|
| PSD (Photoshop) | Layers, adjustment layers, smart objects, masks, effects | Future editing — you can't reconstruct layers from a flat PNG |
| AI (Illustrator) | Vector paths, artboards, live effects, color swatches | Vector editability — rasterization is one-way |
| INDD (InDesign) | Text flow, master pages, styles, linked assets | Document layout editing |
| Camera RAW (CR2, NEF, ARW) | Raw sensor data, full dynamic range (12-14 bits) | Non-destructive photo editing — RAW has 4-16x more data than JPEG |
| FLAC / WAV (audio masters) | Nothing is lost by keeping them | Re-encoding to any lossy format at any bitrate in the future |
| DOCX / ODF (document sources) | Editability, styles, structure | Future revisions — PDF is a dead end for editing |
| Premiere / Final Cut projects | Timeline, effects, transitions, audio mixing | Video re-editing — exported MP4 can't be un-edited |
The workflow: Keep source files in their native format. Export to distribution formats (PNG, PDF, MP4, MP3) when you need to share. Never delete the source because you have the export.
Archival: Convert a Copy, Keep the Original
When converting for archival or format migration, always keep the original alongside the converted version. Conversion is never 100% lossless across all dimensions — even "lossless" conversions can lose metadata, subtleties of the source format's structure, or features the target format doesn't support.
Examples:
- MKV to MP4 may drop subtitle tracks or chapter markers that MKV supports but MP4 doesn't
- DOCX to PDF freezes the layout but loses editability, formulas, and macros
- EPUB to MOBI may lose CSS styling, embedded fonts, or SVG images that MOBI doesn't support
- FLAC to MP3 permanently loses audio data (by design — it's lossy)
Store both: photo.cr2 (original) and photo.tiff (archival copy). document.docx (editable) and document.pdf (distributable). If storage is a concern, archive originals to cold storage (external drive, cloud archive tier) while keeping converted versions for active use.
When the Recipient's Tool Prefers the Current Format
Don't convert a file just to standardize formats if the recipient's workflow expects the current format:
- A print shop requests TIFF — don't convert to PNG "because it's also lossless." Their RIP (Raster Image Processor) is optimized for TIFF and may handle color profiles differently with PNG.
- A developer asks for JSON — don't send YAML "because it's more readable." Their pipeline parses JSON; YAML requires a different parser and introduces quoting/escaping differences.
- A video editor needs ProRes — don't send H.264 "because it's smaller." ProRes is an editing codec designed for timeline scrubbing; H.264 is a delivery codec that's painful to edit with.
- A client sends PSD files — don't convert to PNG for your own convenience and then try to send them back edits in PNG. Work in the format the workflow demands.
The Golden Rules
- Always keep the original. Disk space is cheap; re-creating a lost original is expensive or impossible. Convert copies, never originals.
- Never go lossy→lossy unless you have a good reason. Compatibility is a good reason. "Just in case" is not. If you must, use a higher output bitrate.
- Convert for a purpose, not on principle. Don't convert files "because PNG is better than BMP" unless you actually need the file in PNG. Converting without a purpose just creates duplicate files and potential for confusion.
- Source files stay in source format. PSD stays PSD. RAW stays RAW. The editing format and the distribution format are different, and that's fine.
- When in doubt, don't convert. An unconverted file in a slightly suboptimal format is better than a converted file that lost quality, metadata, or features in the process.
Conversion is a tool, not a goal. The purpose is to transform a file from a format that doesn't serve your needs into one that does. If the current format already serves your needs, the conversion adds nothing and may take something away. The sign of someone who understands file formats isn't that they convert everything — it's that they know when to leave things alone.
Keep your originals, convert copies when there's a clear purpose, and resist the urge to convert for conversion's sake. Your files will thank you.