The extension is a label, not the contents
Renaming photo.png to photo.jpg does not perform a PNG-to-JPG conversion. Software still encounters PNG-encoded data inside the renamed file. A converter decodes the source and creates data that follows the destination format.
Some conversions are lossy
A destination format may discard information to reduce size or because it cannot store a feature from the source. That can affect image detail, audio information, transparency, document structure or metadata.
Containers and codecs are different
Video and audio filenames can hide another layer of complexity. A container such as MP4 can carry streams encoded with different codecs, so two files with the same extension are not necessarily encoded the same way.
Check important outputs
For important documents, media or production assets, inspect the converted result before deleting the source. Conversion is a transformation, and the correct output depends on what you need to preserve.