What is the ruling on exchanging old gold for new gold, with payment of the jeweler's fee, knowing that the conversion process results in a loss of two or three grams from the original gold's weight?

If the jeweler manufactures your jewelry from your own gold and charges a fee for his work, there is no harm in that. However, if he is to make the jewelry for you from gold that is not yours, and takes your gold in exchange for it, along with a fee, then this is impermissible. This is because equality in quantity and immediate possession in the same session are conditions when selling gold for gold of the same type.