Identify the part that must change
A container change and a codec change are different media operations. Remuxing places existing encoded streams into a different supported container. Transcoding decodes and encodes media again. FFmpeg calls the former streamcopy. If the destination accepts the existing codecs but requires another container, re-encoding every frame may be unnecessary. If it rejects the codec itself, a new wrapper will not solve that incompatibility.
Use this workflow for media you own or are authorised to process. Keep the original and write to a new output file. Before choosing an operation, inspect the destination's actual requirements: container, video codec, audio codec and any track restrictions. A filename extension in a help article is less specific than the complete supported-media specification for the target application or device.
A hypothetical delivery decision
Suppose a creator has an authorised recording with video and audio codecs accepted by a training platform, but its container is not accepted. A supported streamcopy into an accepted container may preserve the encoded media without another lossy video generation. That is a sensible candidate for a small test. It is not a universal promise that every codec can be placed into every container.
Now suppose the platform rejects the video codec. Changing only the container can produce a different filename while preserving the incompatible stream. The correct task becomes a documented transcode to the required format. This contrast prevents the common cycle of renaming a file, uploading it again and assuming the platform has ignored the change.
Check what the operation preserves
FFmpeg documents that streamcopy does not decode, filter or encode the selected streams. A picture filter, burned-in caption or pixel rotation therefore requires a different processing path. Track selection and container metadata also need attention: preserving the video does not automatically mean every language, subtitle or attachment will be included in the new file.
Use the tool's documentation for the installed version, because the online FFmpeg documentation follows its newest revision. Review the reported input and output streams before processing a large file. Do not overwrite the only original or combine a format change with unrelated audio and caption edits. A narrow first output is easier to verify and, if necessary, discard.
Accept the result on the intended destination
Test the new file in the actual destination, not only in the preparation computer's most tolerant player. Confirm picture, audio, duration, seeking and the required tracks. A successful mux operation establishes that the tool wrote an output; it does not certify every target player. If the destination still rejects it, retain the exact rejection and output stream information.
The project note should identify the original, selected streams, operation and tested destination. State whether the encoded media was copied or re-encoded, since that distinction matters for later quality decisions. The useful outcome is a compatible authorised deliverable with a traceable processing choice, not a blanket claim that one container is always superior.
Questions answered
Will changing MKV to MP4 always fix playback?
No. The target must support the streams and their combination in that container. A container change cannot add codec support.
Can streamcopy burn captions into the picture?
No. Burning text into video changes decoded picture frames and requires a processing path beyond streamcopy.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.