Define what must change
Start with the target player’s documented requirements and a record of the original file’s tracks. A container mismatch, unsupported video codec and missing audio language are three different problems. Make a protected copy before experimenting and use a separate output filename. There is little value in applying an expensive video conversion when the actual need is to retain a different audio track or change compatible packaging.
Understand the stream-copy route
FFmpeg documents streamcopy as moving selected compressed packets without decoding, filtering or encoding them. That can change a container without a new generation of video compression. It still requires a destination container that can represent the selected streams. The fact that a command uses copy does not mean every possible input can be written into every output format, and it does not add a decoder to the receiving television.
Recognise when decoded processing is required
A change to the picture itself, such as scaling or burning text into the image, requires processing decoded frames rather than simply carrying the original packets across. Re-encoding has choices and consequences, including quality, processing time and compatibility. Decide those from an actual target requirement. Do not label a random preset a lossless fix, or assume that a file becoming smaller proves the incompatibility was correctly addressed.
Verify the output as a new artefact
After a permitted conversion, inspect the output’s video, audio and subtitle inventory and test the required viewing path. Check beginning, middle and ending positions and the tracks that matter to the user. Preserve the original even if the first output seems successful. A conversion can solve one packaging requirement while omitting another stream, and a desktop result remains separate from a real device/account test.