Converting to PNG is the right move for some images and pointless for others. It is worth knowing which you have.
This is the important part. JPG compression discards detail permanently. Converting to PNG stores whatever is left without further loss, but it cannot recover anything, and blockiness or fuzziness that is already in your JPG stays exactly where it is.
Expect the file to grow, often several times over. PNG is lossless, and lossless storage of a photograph is simply bigger. That is a fair trade for a logo or a screenshot and a poor one for a holiday photo.
The real reasons to convert are transparency, which JPG cannot store at all, and editing, since repeatedly saving a JPG degrades it a little each time while PNG does not.
No. It prevents further loss but cannot undo the loss already baked into the JPG.
PNG is lossless. For photographs that means a considerably larger file than the JPG it came from.
Yes.