JPG to PNG is an editing move, not a quality upgrade. PNG stores every pixel losslessly, so once your photo or screenshot is a PNG you can crop, annotate and re-save it repeatedly without the generational blur that creeps into re-saved JPGs. What it cannot do is rebuild detail the original JPG already discarded — the output is exactly as sharp as the input.
This is the right direction when a tool or upload form demands PNG, when you plan to add a transparent background in Photoshop or GIMP, or when you want a pixel-perfect master to work from. Expect the file to grow noticeably — lossless compression of a photographic image is typically three to six times larger than the JPG.