If the chains are gold a jeweler can usually shorten the chains rather easily. This means cutting to length and resoldering the loop (jumpring in jeweler talk) or the chain end back onto the chain.

If silver, some are plated to prevent tarnish but these can be shortened. Keep in mind that with plated silver, there might be a tiny bit of darkening or discoloration at the soldered place. (Only one end generally needs to be cut and resoldered.) Sterling silver with no plated surface can be shortened easily.

Costume jewelry is a different matter. This is for the jeweler to see and decide. Soldering is usually out of the question but there are often other methods to make the chain shorter.

Vermeil, or gold plate over sterling silver is a bear! Any heat and the gold color vanished into thin air! Such chains are shortened by cutting, opening a link and putting in a gold ring with out soldering.

Note on soldering: All chains do not need to be soldered. If the loop the clasp goes into to fasten the chain is not soldered, the jeweler can generally cut the chain and move the loop to the new link for proper length. Soldering is preferred but some chains do not need it.

hope this answers your question.