An image looks rough, so you save it as PNG. The new file is much larger, but the lettering still has the same fuzzy edges. The format changed; the missing information did not return.
Keep an original worth returning to
JPEG uses lossy compression: the saved representation can discard image information. Repeatedly opening, editing and saving as JPEG can introduce further changes. The amount you notice depends on the image, settings and workflow.
Preserve the best available original and create delivery copies from it. For an ongoing editing project, use a format appropriate to the editor that retains the information you need, such as layers, before making a final export.
Match the export to the job
A photograph, a screenshot with small text and a transparent graphic have different needs. Compare a small sample at the size it will actually be viewed. Look at edges, text and gradients as well as the file size.
Avoid choosing solely by extension. Some formats support different compression modes, and export settings matter. Adobe’s compression reference distinguishes lossy and lossless approaches.
Change the workflow, not just the filename
If someone sends a damaged-looking copy, ask for the original or a less compressed export when possible. Converting the existing JPEG to PNG can avoid another lossy encoding at that step, but cannot reconstruct detail already discarded. Keep a clear original-and-export naming scheme so the next edit starts from the right file.